The X-Files as police procedural
[Written for xf_is_love month.]
I've been having some thoughts recently about X-Files as police procedural (and how everything went wrong because of it) and since i completely failed to finish my intended vid for posting today, I thought I'd share some of these thoughts instead.
(I should note at this point that although I am a total mine of useless information when it comes to British crime fiction - quite apart from the life-long love thing, it formed the basis of my Masters thesis - my knowledge of US crime fiction is patchy at best, and my knowledge of crime fiction outside of the UK and US can only be described as shameful. So I fully acknowledge there's some gaps, especially when thinking about the X-Files in a US television context - so if you have any corrections/contradictions/better examples, please let me know!)
So, crime dramas. The backbone of TV schedules the world over. And why not? The mystery angle provides an instant hook; if you get the casting right and your leads have chemistry then you'll probably get a loyal following of dedicated viewers; and best of all, each episode is self-contained, meaning you can appeal to the casual viewers who can tune in and out and not have to worry if they miss a few episodes. Easy to see why tv networks bloody love crime dramas and would probably fill the schedules with nothing else if they could - they're (relatively) cheap, they're reliable, and if you hit on a good formula you can run them for years and years.
But with so many crime dramas clogging up the schedules, how do you make a new one stand out? Usually, by finding some sort of twist or quirk on the basic premise - your sleuth is…psychic! Magic! A time traveller! A pie-maker who brings people back from the dead! Canadian! (Wait, no, not just Canadian, but a Mountie! With a hat! And a wolf!) No, scratch that, a little old lady! A vicar! A society lady in the '30s! A gangster in Birmingham in the '20s! Solving murders during WWII! ALL OF THE ABOVE! (You'd watch the hell out of that show, wouldn't you?)
Or…you take the X-Files route, where the selling point is not the uniqueness of the investigator(s) solving the mysteries, but the mysteries themselves - in this case, adding in some science-fiction and fantasy twists. It was nowhere near the first show to combine supernatural elements with a crime drama - but I would argue, one of the first (the first, maybe?) to do so while still being pure straight police procedural in format. (Except when it wasn't, which is kind of my point - I'll get to that later.)
Do I need to defend X-Files as 'crime drama' instead of science-fiction/fantasy show here, btw? I think it's a fairly standard reading, but very briefly - in case anyone wants to object - I view the X-Files as pretty much a pure crime drama in format, albeit one with supernatural elements laid on top to up the creep factor, because it follows the rules and conventions of crime drama, particularly the police procedural. (And when it doesn't follow those rules, that's when everything goes horribly wrong…) The rules for a SFF show are different; broadly speaking, in my opinion, a SFF show is about setting up a defined universe - a different world with its own internally consistent set of rules - and then telling stories within it. Where the X-Files falls down on the SFF criteria for me is that pesky 'internally consistent' thing. The creators didn't start out with a world with even a loosely-defined set of rules and then start telling stories within that world, they started telling stories with zero connection to each other and then later tried to tie it at least some of it together. I'll leave it up to your own judgement whether they succeeded or not.
And yeah, okay, I'm just arguing semantics here and plenty of shows - including the X-Files - crossover genres, or borrow bits and pieces from different genres. Normally it doesn't matter at all what kind of label you slap on a show - but since I'm about to argue that messing with the rules of police procedurals is what ultimately went wrong with the X-Files, it matters to me in this instance.
So. X-Files as crime drama - or rather, the specific subset of crime drama that is the police procedural, where essentially the focus of an episode is the mystery to be solved rather than the lives of our detectives. Think any cop show where 90-100% of the main characters' time is spent asking questions so that guest characters can spout exposition at them, and the other 0-10% is spent on a subplot that doesn't have any connection to the central story being told.
Though actually, I'm now going to throw out the term 'police procedural' altogether as it's a bit messy - and I'm going to take a brief detour into structuralism and the work of a Franco-Bulgarian theorist called Tzvetan Todorov (bear with me). See, in around the 60s and 70s, there was a bit of a fashion in literary theory for basically identifying and writing down the patterns found in different types of fiction - and in 1966, Todorov wrote an article called 'The Typography of Detective Fiction' which he explored the underlying structures of crime fiction in particular. Most of it's completely irrelevant here, but I am going to nick his terms for the different types of detective fiction; namely, the 'whodunit' and the thriller.
I'm going to be really, really lazy here and copy and paste some bits from an essay I wrote several years ago on this topic (so forgive the 'literary theory essay' vibe of the following, bolding is mine):
"One of the…key concepts to come out of Russian formalism was the idea of story vs. plot; that is, the distinction between fabula or "story" (the actual events, in chronological order, as presented in the tale) and sjuzet (also spelt syuzhet) or "plot" (the order and manner in which the tale is told)….
Todorov [outlines] three types of detective fiction - the 'whodunit', the thriller, and the suspense novel - taking as his basis for differentiation the basic formalist idea of 'story vs. plot'. In the whodunit, the 'story' (the crime) has usually taken place before the novel begins and is never described directly to the reader, whilst the 'plot' (the investigation) is the tale actually related to the reader. There is therefore a complete separation between story and plot. The 'second story', as Todorov calls it, of the investigation is only the process by which the 'first story' is assembled by both the detective and the reader; it has no importance or significance of its own. As a result, the detective is immune from physical danger throughout and does not change or grow himself. His only role within the present story (the 'plot') is to learn the past story of the crime (the 'story').
The thriller, in contrast, does not have this separation between story and plot, but rather merges the two together. The story of the crime is the catalyst for the events of the plot, rather than the central mystery. The protagonist is usually more heavily involved in the plot than in the whodunit; he is placed in physical danger and there is a risk that he may not even make it to the end of the story alive.
The third form of detective fiction Todorov identifies is the suspense novel, which falls between the whodunit and the thriller, combining elements of each. The suspense novel retains the separation of story and plot of the whodunit - the 'plot' is concerned with solving the central mystery of the 'story' - but the 'plot' is more significant to the reader, as in the thriller. As with the thriller, the detective does not have immunity and may put themselves in personal danger in the investigation of the crime.
The main differences between the three types therefore relate to the position of the reader. In the whodunit, the sole interest for the reader is in the past event of the crime (the story); in the thriller, the interest is in the present events of the investigation and the rest of the plot as it unfolds; and in the suspense novel, the interest is in both the past event of the crime and the present events of the investigation."
Replacing novel with episode, and reader with audience, what you basically have there is the difference between a police procedural (whodunit) and thriller - in the former, the investigation is how the 'real' story of the episode is revealed to us, and in the latter, the investigation is the story - the dead body, or whatever it is, is only the catalyst for the action to start. These days, most crime novels fall somewhere between the two - what Todorov calls the 'suspense' novel - and most TV shows do as well, but as a sliding scale it can be pretty useful to differentiate between the two types. For instance, Elementary vs. BBC's Sherlock; the former is basically a whodunit, where the mystery is the focus in each episode, while Sherlock takes more of a 'thriller' (…well, soap opera) approach, particularly in Series 3, where the mystery is almost irrelevant to each episode as it's really about the main characters' lives.
A 'pure' whodunit, or procedural, is pretty rare on television and actually always has been. Audiences like to invest in their main characters, so a little bit of physical danger, or our detectives' personal lives getting mixed up with a case of the week is generally par for the course. But what marks out a whodunit/procedural is the reset button - the knowledge that okay, in this episode Starsky may shed tears over his dead ex-girlfriend, or Lewis may mourn his dead ex-sergeant, or Morse may regret getting involved with yet another murderer (…I really need to make that 'women in detective fiction' vid…), but everything will be back to normal next week and that character will never be mentioned again.
Remember how in the first series of the X-Files, an astonishing number of Mulder and Scully's old friends, ex-partners and ex-lovers turned up, usually to die by episode end? That's because at its heart, and especially in the early days, the X-Files was pure police procedural, and that's just what happens in those kind of cop shows. Okay, so the solution to the mystery was sometimes 'we don't really have a clue what just happened, but hey, roll credits!', and the actual 'criminal' was usually aliens, or ghosts, or vampires, or vampire ghosts…but in all other senses, it followed all the rules. The mystery was wrapped up each week, once it was wrapped up that particular mystery never got mentioned again, and the focus of every episode was the investigation of the X-File itself, not our intrepid investigators.
Of course, having established this formula, they almost immediately broke it - because that's what good TV shows do, right, mix things up? Well…yes and no. By the end of the first series, the X-Files is closed down and the status quo is no more. Then in series 2, things get really ripped up, with Scully's abduction and everything it later led to. And the thing is, that initial decision was forced on the creators by external circumstances - Gillian's pregnancy, and uncertainty over whether they were going to get another series. We don't know what direction the show might have taken without the external factors - the boys at 1013 might have started ripping up the formula at some point anyway - or they might have stuck with the pure whodunits and never established any kind of myth arc at all.
It would take a brave X-Files fan to say they would have been better off without the myth arc at all, and it's not going to be me - the myth arc gave us some of my all-time favourite episodes, and probably fuelled like 99% of my obsessive fannishness back in the day. (I knew Mulder couldn't really be dead…but my god, I couldn't wait for the next season.) And yet, I'm going to take a stand here and say that establishing the myth arc - throwing in 'thriller' elements to a show that was pure 'whodunit' - was what broke the X-Files and caused its inevitable downfall.
Granted, no show lasts forever, and you'll never get any kind of agreement among X-Files fans about when exactly the show became Not Good, or that it even declined to that extent at all (there's plenty of fans of Seasons 8 & 9 out there and I totally respect that), and even in my own personal view of things there's a good couple of years between breaking the whodunit format and the problems it later led to. And bending the rules of genres can be a good thing, if you're consistent with it and follow through. And yet…I would argue that everything that went wrong in later years comes from breaking that format in Season 2. I don't mean the odd bad episode, I mean the deep, structural problems.
Audiences have expectations. I mentioned Starsky up above, so okay, let's take Starsky and Hutch as an example of a whodunit tv show - when we watch it, we know what to expect. Bad stuff will happen to our central characters and the people they care about, but it will only matter in that episode. By next week - or the week after, if it's a 2-parter - it will be quite literally as though it never happened. Indeed, S&H used to finish every episode with a light-hearted scene - an episode tag - almost to reassure the viewers that okay, it got angsty and emotional tonight, but look, everything's back to normal now and we start all over again next week - same time, same channel! Because that's what happens in these kind of police procedural shows. The audience doesn't feel cheated by it, or have any kind of expectation that these traumatic life events will be followed up on, because they know the rules of the genre that they're watching.
A different kind of genre leads to different expectations. If you're watching a drama that actually focuses on its main characters (there isn't a nice catch-all term for them, though in my head I think of them all as soap operas quite frankly), then the audience will expect that actions will have consequences. There will some kind of over-arching plot. Okay, it might not be good, or even that well planned-out, but it will be followed through. If character X is having money troubles, they're only going to get worse episode by episode until X does something desperate that leads to the next plot turn. If Buffy lies to her friends about something, that's going to come back and bite her on the ass in a few episodes' time. And again, the audience knows these rules, they know what to expect.
Where the X-Files went wrong was that it tried to have its cake and eat it too. It was a police procedural - a 'whodunit' - and it tried to stay a whodunit at the same time as becoming a thriller. The show introduced recurring elements; it introduced an overarching conspiracy that got a bit less mysterious every time we saw it; it started actually following up on things that had happened in previous seasons; and most importantly, it stopped just telling the story of each individual X-File, and started telling the story of 'Mulder and Scully'. A few times a year, anyway. Which would have been okay, actually, had they been consistent with it - but they tried to keep it as a police procedural as well.
What they actually ended up doing was splitting the show in two, and to be fair, that worked quite well for a few years, with the 'monster of the week' episodes sticking to the whodunit format, and the myth arc episodes following more of a thriller format. But it wasn't sustainable. You can't run two different TV shows at the same time. They couldn't commit fully to the 'thriller' side of the show, because it wasn't the point of the show - for most of the year, the X-Files was exactly as it always had been, a series of stand-alone supernatural mysteries about the Monster of the Week, not about the agents investigating them. As brilliant as the myth arc episodes were for a few years, eventually, the myth arc began to sink under its own weight of convoluted plot twists and missed opportunities, while the Monsters of the Week carried on merrily, almost in a totally different show.
It ended up in disaster, as it was always going to. When exactly that disaster occurred is a question that's largely a matter of personal taste, and this post is getting long enough already so I won't go into all the permutation here. For me, the problems set in in Series 4, with Scully's cancer arc - something you simply can't do in a pure whodunit show without causing massive tonal whiplash - but the quality of the show in general, the acting, writing and direction, was high enough to cover over the cracks for a few seasons yet. No, the point where it totally crashed and burned for me was Series 8. (Your Mileage May Vary.) Entirely because of this problem of trying to be a whodunit and a thriller at the same time.
You can do cast changes well in a whodunit series - the focus is on the mysteries themselves, so as long as the mysteries remain engaging and the new detective team is likeable enough, you'll probably get away with it. And you can do cast changes well in a thriller (or any kind of soapy series), because it's a world where actions have consequences so we get to see the remaining characters react to their loss and to the new person in their midst, much as the audience is reacting to it.
What you can't do is cast changes well in a TV show where the show runners are picking and choosing the worst elements of both formats. You can't set up a situation where your main characters get to have an emotional response to the loss of another character, along with setting up various dangling plot threads, then completely ignore everything you just introduced in favour of a series of whodunit episodes. And you really, really can't tell a pregnancy story for one of your main characters as a (ugh, literal) whodunit. Because then the audience will feel cheated. They no longer know what to expect; they no longer know which actions will be followed by consequences and which won't. More importantly, they no longer care.
That's not to say crossing genres is a bad thing, or that a show shouldn't grow and change from its original premise. You can absolutely change from one format to another - I would argue that Smallville for instance was very successful in turning from a Monster of the Week style format to a comic book soap opera. The transition period was, I'll grant you, a little rough, but once they settled on the new formula they followed through with it and that's what worked. I don't watch Person of Interest, but I've heard it was pretty successful in moving from a standalone whodunit style format to more of a thriller/myth arc style - because again, they followed through with the new elements they introduced.
I love detective shows. I love police procedurals. I love thrillers. (Honest confession time: I also love soap operas.) And I loved the X-Files, a lot - I can find many episodes I love in every single season, even 8 & 9. But as much as I loved it, part of me will always weep for the way it destroyed itself. You can't be a servant to two masters; and you can't be two opposite types of crime fiction at the same time.
(That…got a lot longer than intended, oops. Apologies for any typos, I need to post this and run. Congrats to anyone who made it to the end!)
I've been having some thoughts recently about X-Files as police procedural (and how everything went wrong because of it) and since i completely failed to finish my intended vid for posting today, I thought I'd share some of these thoughts instead.
(I should note at this point that although I am a total mine of useless information when it comes to British crime fiction - quite apart from the life-long love thing, it formed the basis of my Masters thesis - my knowledge of US crime fiction is patchy at best, and my knowledge of crime fiction outside of the UK and US can only be described as shameful. So I fully acknowledge there's some gaps, especially when thinking about the X-Files in a US television context - so if you have any corrections/contradictions/better examples, please let me know!)
So, crime dramas. The backbone of TV schedules the world over. And why not? The mystery angle provides an instant hook; if you get the casting right and your leads have chemistry then you'll probably get a loyal following of dedicated viewers; and best of all, each episode is self-contained, meaning you can appeal to the casual viewers who can tune in and out and not have to worry if they miss a few episodes. Easy to see why tv networks bloody love crime dramas and would probably fill the schedules with nothing else if they could - they're (relatively) cheap, they're reliable, and if you hit on a good formula you can run them for years and years.
But with so many crime dramas clogging up the schedules, how do you make a new one stand out? Usually, by finding some sort of twist or quirk on the basic premise - your sleuth is…psychic! Magic! A time traveller! A pie-maker who brings people back from the dead! Canadian! (Wait, no, not just Canadian, but a Mountie! With a hat! And a wolf!) No, scratch that, a little old lady! A vicar! A society lady in the '30s! A gangster in Birmingham in the '20s! Solving murders during WWII! ALL OF THE ABOVE! (You'd watch the hell out of that show, wouldn't you?)
Or…you take the X-Files route, where the selling point is not the uniqueness of the investigator(s) solving the mysteries, but the mysteries themselves - in this case, adding in some science-fiction and fantasy twists. It was nowhere near the first show to combine supernatural elements with a crime drama - but I would argue, one of the first (the first, maybe?) to do so while still being pure straight police procedural in format. (Except when it wasn't, which is kind of my point - I'll get to that later.)
Do I need to defend X-Files as 'crime drama' instead of science-fiction/fantasy show here, btw? I think it's a fairly standard reading, but very briefly - in case anyone wants to object - I view the X-Files as pretty much a pure crime drama in format, albeit one with supernatural elements laid on top to up the creep factor, because it follows the rules and conventions of crime drama, particularly the police procedural. (And when it doesn't follow those rules, that's when everything goes horribly wrong…) The rules for a SFF show are different; broadly speaking, in my opinion, a SFF show is about setting up a defined universe - a different world with its own internally consistent set of rules - and then telling stories within it. Where the X-Files falls down on the SFF criteria for me is that pesky 'internally consistent' thing. The creators didn't start out with a world with even a loosely-defined set of rules and then start telling stories within that world, they started telling stories with zero connection to each other and then later tried to tie it at least some of it together. I'll leave it up to your own judgement whether they succeeded or not.
And yeah, okay, I'm just arguing semantics here and plenty of shows - including the X-Files - crossover genres, or borrow bits and pieces from different genres. Normally it doesn't matter at all what kind of label you slap on a show - but since I'm about to argue that messing with the rules of police procedurals is what ultimately went wrong with the X-Files, it matters to me in this instance.
So. X-Files as crime drama - or rather, the specific subset of crime drama that is the police procedural, where essentially the focus of an episode is the mystery to be solved rather than the lives of our detectives. Think any cop show where 90-100% of the main characters' time is spent asking questions so that guest characters can spout exposition at them, and the other 0-10% is spent on a subplot that doesn't have any connection to the central story being told.
Though actually, I'm now going to throw out the term 'police procedural' altogether as it's a bit messy - and I'm going to take a brief detour into structuralism and the work of a Franco-Bulgarian theorist called Tzvetan Todorov (bear with me). See, in around the 60s and 70s, there was a bit of a fashion in literary theory for basically identifying and writing down the patterns found in different types of fiction - and in 1966, Todorov wrote an article called 'The Typography of Detective Fiction' which he explored the underlying structures of crime fiction in particular. Most of it's completely irrelevant here, but I am going to nick his terms for the different types of detective fiction; namely, the 'whodunit' and the thriller.
I'm going to be really, really lazy here and copy and paste some bits from an essay I wrote several years ago on this topic (so forgive the 'literary theory essay' vibe of the following, bolding is mine):
"One of the…key concepts to come out of Russian formalism was the idea of story vs. plot; that is, the distinction between fabula or "story" (the actual events, in chronological order, as presented in the tale) and sjuzet (also spelt syuzhet) or "plot" (the order and manner in which the tale is told)….
Todorov [outlines] three types of detective fiction - the 'whodunit', the thriller, and the suspense novel - taking as his basis for differentiation the basic formalist idea of 'story vs. plot'. In the whodunit, the 'story' (the crime) has usually taken place before the novel begins and is never described directly to the reader, whilst the 'plot' (the investigation) is the tale actually related to the reader. There is therefore a complete separation between story and plot. The 'second story', as Todorov calls it, of the investigation is only the process by which the 'first story' is assembled by both the detective and the reader; it has no importance or significance of its own. As a result, the detective is immune from physical danger throughout and does not change or grow himself. His only role within the present story (the 'plot') is to learn the past story of the crime (the 'story').
The thriller, in contrast, does not have this separation between story and plot, but rather merges the two together. The story of the crime is the catalyst for the events of the plot, rather than the central mystery. The protagonist is usually more heavily involved in the plot than in the whodunit; he is placed in physical danger and there is a risk that he may not even make it to the end of the story alive.
The third form of detective fiction Todorov identifies is the suspense novel, which falls between the whodunit and the thriller, combining elements of each. The suspense novel retains the separation of story and plot of the whodunit - the 'plot' is concerned with solving the central mystery of the 'story' - but the 'plot' is more significant to the reader, as in the thriller. As with the thriller, the detective does not have immunity and may put themselves in personal danger in the investigation of the crime.
The main differences between the three types therefore relate to the position of the reader. In the whodunit, the sole interest for the reader is in the past event of the crime (the story); in the thriller, the interest is in the present events of the investigation and the rest of the plot as it unfolds; and in the suspense novel, the interest is in both the past event of the crime and the present events of the investigation."
Replacing novel with episode, and reader with audience, what you basically have there is the difference between a police procedural (whodunit) and thriller - in the former, the investigation is how the 'real' story of the episode is revealed to us, and in the latter, the investigation is the story - the dead body, or whatever it is, is only the catalyst for the action to start. These days, most crime novels fall somewhere between the two - what Todorov calls the 'suspense' novel - and most TV shows do as well, but as a sliding scale it can be pretty useful to differentiate between the two types. For instance, Elementary vs. BBC's Sherlock; the former is basically a whodunit, where the mystery is the focus in each episode, while Sherlock takes more of a 'thriller' (…well, soap opera) approach, particularly in Series 3, where the mystery is almost irrelevant to each episode as it's really about the main characters' lives.
A 'pure' whodunit, or procedural, is pretty rare on television and actually always has been. Audiences like to invest in their main characters, so a little bit of physical danger, or our detectives' personal lives getting mixed up with a case of the week is generally par for the course. But what marks out a whodunit/procedural is the reset button - the knowledge that okay, in this episode Starsky may shed tears over his dead ex-girlfriend, or Lewis may mourn his dead ex-sergeant, or Morse may regret getting involved with yet another murderer (…I really need to make that 'women in detective fiction' vid…), but everything will be back to normal next week and that character will never be mentioned again.
Remember how in the first series of the X-Files, an astonishing number of Mulder and Scully's old friends, ex-partners and ex-lovers turned up, usually to die by episode end? That's because at its heart, and especially in the early days, the X-Files was pure police procedural, and that's just what happens in those kind of cop shows. Okay, so the solution to the mystery was sometimes 'we don't really have a clue what just happened, but hey, roll credits!', and the actual 'criminal' was usually aliens, or ghosts, or vampires, or vampire ghosts…but in all other senses, it followed all the rules. The mystery was wrapped up each week, once it was wrapped up that particular mystery never got mentioned again, and the focus of every episode was the investigation of the X-File itself, not our intrepid investigators.
Of course, having established this formula, they almost immediately broke it - because that's what good TV shows do, right, mix things up? Well…yes and no. By the end of the first series, the X-Files is closed down and the status quo is no more. Then in series 2, things get really ripped up, with Scully's abduction and everything it later led to. And the thing is, that initial decision was forced on the creators by external circumstances - Gillian's pregnancy, and uncertainty over whether they were going to get another series. We don't know what direction the show might have taken without the external factors - the boys at 1013 might have started ripping up the formula at some point anyway - or they might have stuck with the pure whodunits and never established any kind of myth arc at all.
It would take a brave X-Files fan to say they would have been better off without the myth arc at all, and it's not going to be me - the myth arc gave us some of my all-time favourite episodes, and probably fuelled like 99% of my obsessive fannishness back in the day. (I knew Mulder couldn't really be dead…but my god, I couldn't wait for the next season.) And yet, I'm going to take a stand here and say that establishing the myth arc - throwing in 'thriller' elements to a show that was pure 'whodunit' - was what broke the X-Files and caused its inevitable downfall.
Granted, no show lasts forever, and you'll never get any kind of agreement among X-Files fans about when exactly the show became Not Good, or that it even declined to that extent at all (there's plenty of fans of Seasons 8 & 9 out there and I totally respect that), and even in my own personal view of things there's a good couple of years between breaking the whodunit format and the problems it later led to. And bending the rules of genres can be a good thing, if you're consistent with it and follow through. And yet…I would argue that everything that went wrong in later years comes from breaking that format in Season 2. I don't mean the odd bad episode, I mean the deep, structural problems.
Audiences have expectations. I mentioned Starsky up above, so okay, let's take Starsky and Hutch as an example of a whodunit tv show - when we watch it, we know what to expect. Bad stuff will happen to our central characters and the people they care about, but it will only matter in that episode. By next week - or the week after, if it's a 2-parter - it will be quite literally as though it never happened. Indeed, S&H used to finish every episode with a light-hearted scene - an episode tag - almost to reassure the viewers that okay, it got angsty and emotional tonight, but look, everything's back to normal now and we start all over again next week - same time, same channel! Because that's what happens in these kind of police procedural shows. The audience doesn't feel cheated by it, or have any kind of expectation that these traumatic life events will be followed up on, because they know the rules of the genre that they're watching.
A different kind of genre leads to different expectations. If you're watching a drama that actually focuses on its main characters (there isn't a nice catch-all term for them, though in my head I think of them all as soap operas quite frankly), then the audience will expect that actions will have consequences. There will some kind of over-arching plot. Okay, it might not be good, or even that well planned-out, but it will be followed through. If character X is having money troubles, they're only going to get worse episode by episode until X does something desperate that leads to the next plot turn. If Buffy lies to her friends about something, that's going to come back and bite her on the ass in a few episodes' time. And again, the audience knows these rules, they know what to expect.
Where the X-Files went wrong was that it tried to have its cake and eat it too. It was a police procedural - a 'whodunit' - and it tried to stay a whodunit at the same time as becoming a thriller. The show introduced recurring elements; it introduced an overarching conspiracy that got a bit less mysterious every time we saw it; it started actually following up on things that had happened in previous seasons; and most importantly, it stopped just telling the story of each individual X-File, and started telling the story of 'Mulder and Scully'. A few times a year, anyway. Which would have been okay, actually, had they been consistent with it - but they tried to keep it as a police procedural as well.
What they actually ended up doing was splitting the show in two, and to be fair, that worked quite well for a few years, with the 'monster of the week' episodes sticking to the whodunit format, and the myth arc episodes following more of a thriller format. But it wasn't sustainable. You can't run two different TV shows at the same time. They couldn't commit fully to the 'thriller' side of the show, because it wasn't the point of the show - for most of the year, the X-Files was exactly as it always had been, a series of stand-alone supernatural mysteries about the Monster of the Week, not about the agents investigating them. As brilliant as the myth arc episodes were for a few years, eventually, the myth arc began to sink under its own weight of convoluted plot twists and missed opportunities, while the Monsters of the Week carried on merrily, almost in a totally different show.
It ended up in disaster, as it was always going to. When exactly that disaster occurred is a question that's largely a matter of personal taste, and this post is getting long enough already so I won't go into all the permutation here. For me, the problems set in in Series 4, with Scully's cancer arc - something you simply can't do in a pure whodunit show without causing massive tonal whiplash - but the quality of the show in general, the acting, writing and direction, was high enough to cover over the cracks for a few seasons yet. No, the point where it totally crashed and burned for me was Series 8. (Your Mileage May Vary.) Entirely because of this problem of trying to be a whodunit and a thriller at the same time.
You can do cast changes well in a whodunit series - the focus is on the mysteries themselves, so as long as the mysteries remain engaging and the new detective team is likeable enough, you'll probably get away with it. And you can do cast changes well in a thriller (or any kind of soapy series), because it's a world where actions have consequences so we get to see the remaining characters react to their loss and to the new person in their midst, much as the audience is reacting to it.
What you can't do is cast changes well in a TV show where the show runners are picking and choosing the worst elements of both formats. You can't set up a situation where your main characters get to have an emotional response to the loss of another character, along with setting up various dangling plot threads, then completely ignore everything you just introduced in favour of a series of whodunit episodes. And you really, really can't tell a pregnancy story for one of your main characters as a (ugh, literal) whodunit. Because then the audience will feel cheated. They no longer know what to expect; they no longer know which actions will be followed by consequences and which won't. More importantly, they no longer care.
That's not to say crossing genres is a bad thing, or that a show shouldn't grow and change from its original premise. You can absolutely change from one format to another - I would argue that Smallville for instance was very successful in turning from a Monster of the Week style format to a comic book soap opera. The transition period was, I'll grant you, a little rough, but once they settled on the new formula they followed through with it and that's what worked. I don't watch Person of Interest, but I've heard it was pretty successful in moving from a standalone whodunit style format to more of a thriller/myth arc style - because again, they followed through with the new elements they introduced.
I love detective shows. I love police procedurals. I love thrillers. (Honest confession time: I also love soap operas.) And I loved the X-Files, a lot - I can find many episodes I love in every single season, even 8 & 9. But as much as I loved it, part of me will always weep for the way it destroyed itself. You can't be a servant to two masters; and you can't be two opposite types of crime fiction at the same time.
(That…got a lot longer than intended, oops. Apologies for any typos, I need to post this and run. Congrats to anyone who made it to the end!)