Choose ten characters. What fandoms would they participate in, and in what ways?
1) Harry Dresden from The Dresden Files follows Spider-Man religiously--well, it practically IS a religion for him, since he says in canon that he follows "the Tao of Peter Parker" (with great power comes great responsibility). He's been collecting the comics since he was ten or so, has attended a couple of comic book conventions, and has, on occasion, submitted long typewritten letters to Marvel wrangling with Joe Quesada on the subject of the One More Day storyline, because he HATED the idea of Spider-Man losing the woman he loved because of the forces of evil. That was just a bit too close to home.
2) Peter Pettigrew from Harry Potter would be a fan of hard science fiction. If he had been a Muggle kid, I think he'd have liked fantasy, but since his world is fantasy, he'd look for what the wizarding world doesn't have...technology. I can see him playing an Doctor Who-based RPG-by- mail-and-owl-post with a lot of other Muggleborn kids who attend other schools or who are being homeschooled. And their non-magical siblings, too. And he's totally tried to explain Doctor Who and Star Wars to his friends. It's just never worked.
3) Lara Raith from The Dresden Files? Easy. She follows the Twilight fandom--not because she likes it, but because, unbeknownest to Meyer, she commissioned the series. What better way to convince an entire generation that vampires are not monsters, but beautiful and honorable creatures, worthy of great respect, and that any human should regard it as a joy to associate with them? She also posts fanfic about Twilight vampires--generally on Adultfanfiction.net--and fanvids on YouTube.
In her spare time, she trolls religious and mythology websites and does her level best to erase any knowledge about demons and eldritch abominations as soon as the topic comes up.
4) Molly Carpenter from The Dresden Files follows several bandoms--I'm not sure which, but My Chemical Romance and Panic at the Disco might be among them--and writes RPF or RPS in all of them. She doesn't think her mother knows anything about fanfic, much less RPS, but she does her very best to hide all of her stories from her father and from Harry. She's not really sure how she'd explain her stories to them.
5) Annie Sawyer from Being Human follows Hollyoaks. If she were solid, she'd write romantic fanfic. Since she's not, she talks out the stories to George. She's tried talking about the soaps to Mitchell, but Mitchell just stares. He's not much for romances.
6) Sansa Stark from A Song of Ice and Fire canonically IS a fangirl of medieval-style ballads, heroic sagas and fairy tales. I think that she would try to make up new stanzas for her favorite ballads and add characters to stories--such as brave princesses and heroes who can see their worth and love them.
7) DEATH from Discworld would favor two genres in particular: Mysteries With Cats and Romances With Cats. I'm quite sure that he reads for the cats and not for the stories themselves. I'm also sure that he would try to write such stories himself and fail rather miserably. I'm also sure that he has a certain fascination with his universe's version of Sandman, though he is curious about why the author portrayed him as a pretty, perky girl who likes the color black, collects hats, and has six siblings and two pet goldfish.
8) Neal Caffrey from White Collar loves action-adventure everything. He doesn't write stories, though; he makes vids, which he considers "instructional videos for people who are tired of boring lives." He even did that in prison, much to the chagrin of the FBI in general and Peter in particular, because Neal's vids never demonstrated anything illegal; they left you on the step before that.
Also, Neal adores books and TV shows that feature complex clues and conspiracies, and has spent a fair amount of time online arguing that a) Dan Brown is an unappreciated genius and b) The X-Files is far more plausible in its depiction of how the government would approach cases involving the paranormal than anyone wants to think. Conversely, he refuses to take shows like Ghost Hunters and Ghost Lab seriously, because he knows exactly how to fake all of the effects that the shows say might be ghosts.
9) Abby Sciuto from NCIS canonically loves things that are considered creepy, and in her off-hours, she draws Addams Family and Edward Gorey-esque pictures which she posts on DeviantArt, writes and performs filk songs which she posts on YouTube, and writes Repo: The Genetic Opera fanfic. Her one grief is that she can't write horror at all; most of her stuff, despite the subject matter, is very funny.
10) The Doctor from Doctor Who. I'll go through the list of his various incarnations:
One wrote historical AUs...which often weren't AUs, as far as he was concerned, but simply a record of the truth. He used to get quite testy about that.
Two tried to compose music about his adventures for the flute and wrote fanfic about Charlie Chaplin movies, Bonnie Prince Charlie and slice-of-life tales set in 1960s England.
Three wrote long, sad original science fiction about people who were alien within their own societies or who were forced to live in a completely alien culture. He also made insanely complex science fiction miniatures that actually worked.
Four didn't bother to write--his attention span was too short and he was back in space again--but he did quite a lot of knitting and crocheting records of his own adventures into impossibly long scarves. (His adventures were long, so he never really knew where to stop.)
Five wrote Real People Fic about cricketers, usually illustrated with photographs of the cricketers on the pitch.
Six loved writing sarcastic novellas based on Heinlein and Harlan Ellison stories and self-insert fic and plays based on EastEnders.
Seven loved intricate and Machiavellian plots, both in books and in movies. It's a tossup whether his best stories were about Havelock Vetinari or George Smiley. He also dearly loved action-adventure stories with heroes and heroines who didn't look the part. (He adored Gabrielle from Xena, for example.) And he loved to go on various forums, find the most outrageous theory there, and spin a tale to make it sound convincing.
Eight was a fan of 18th and 19th century literature, so his stories and paintings tended to be based on period books (such as Austen, Dickens and Tolstoy), BBC costume dramas, Masterpiece Theatre and Merchant-Ivory films.
Nine liked black comedy, probably because not a lot struck him as funny after the Time War. He wrote scads of stories set in the M*A*S*H universe and in the Hitchhiker's Guide one. He also wrote tons of stories based on Dickens' novels, but never posted them anywhere. He also collected everything available about Dickens and his work.
Finally, he was enough of a Harry Potter fan to take the TARDIS to the Potterverse and pick up Lupin after James and Lily's deaths--well, why not? The TARDIS goes everywhere, including to fictional universes. Remus and he traveled together for some time (that split second between his leaving Rose and coming back for her was quite a bit longer for him than for her), and it was from that that he gained his later appreciation for werewolves.
Ten was, canonically, a Harry Potter and Shakespeare fan. He tried his hand at writing Shakespearean sonnets and poems in iambic pentameter (he was pretty good, by the way) and posted Harry Potter stories and essays everywhere.
He also tried posting stories about Remus Lupin traveling with aliens during the Lost Years, but the HP fans found the idea patently ridiculous, and eventually he dropped the series.