Okay, so my current WIP is being written in 3rd-person limited POV where I have the opportunity to switch POV from various characters depending on the scene. I've found it interesting to explore the characters' voices in doing so, and decided to do up a list of the different characters' voice in terms of how easy it is for me to write. Going from easiest to hardest.
Note that I'm just speaking from my experience writing this particular fic, which is set shortly after Chosen. So no Spike, but there is William.
1. Xander - I suppose this shouldn't be a big surprise that Xander's fairly easy to write for. He's the Everyman. The funny guy. Come up with some creative jokes, mix it with pop culture references, add a little Buffy-worship, and there's Xander. Also, considering Xander and I deal with conflict in much the same way (Joke about it!), it makes it frightfully easy to gauge his reactions and write appropriately.
2. Giles - Oddly enough, My normal, formal voice sounds a lot like I imagine Giles' internal voice does. For Giles, I find myself thinking about the situation in a logical manner and then simply wording things as I would tend to word them if writing an essay (Keeping out most humorous asides). I think, especially by the end of the series, it's important to portray Giles as being weary. He's tired. He wants to continue to help Buffy, yet he also wants to let her handle things on her own. He's been fighting the good fight for a long time, and he'll continue to do so as long as he's needed. But he doesn't necessarily want to do it anymore.
3. Willow - There's a reason I use Willow for most of my icons. I easily fall into "Willow-speak" in RL. She has a unique way of talking and thinking. It can be hard to get a grasp on her character as she doesn't talk about herself very much. However, after the events of S6, she's a bit easier to understand. The main thing I keep in mind with Willow is that she wants social harmony. She can't stand for her friends to be fighting or unhappy. She wants people to be happy. And, while she's learned not to resort to magic to keep the status quo, she'll still do what she can to keep them happy or express frustration when they're not.
4. Buffy - Buffy can be a difficult character to get a handle on. She's subtle, though she goes through so much development over the course of seven seasons. She may lapse into peppy "Buffy-speak" in her internal dialogue at times, though I don't think that would happen as often as it might in a fic set earlier in the series. Instead, she's a bit more resigned to her destiny and to the pressures of her responsibilities. I don't so much have trouble getting Buffy's voice down as I do figuring out her actions. It can be difficult for me to try to predict what Buffy might do, and I have to think carefully about what she's done in the past and if she's learned from that or if she would still do it. Plus, Buffy is torn between what she wants to do and what she feels she has to do as the Slayer.
5. William - It should be obvious why this is difficult. Figuring out the voice of a Victorian gentleman just isn't easy. I looked through a lot of newspaper articles and letters from that time period to get a feel for how they spoke (Or wrote, at least). I try to mirror that with Willliam's thoughts, though I admit that I probably fall short quite often (That'll be edited a lot). The main trouble I've run into with William is judging his reaction to things, for obvious reasons. What on earth would a guy from the later 1800s think of television? How would he think about it? Also, I have to carefully keep slang terms in check when writing from his POV. At the same time, I don't want his character to fall into being a stereotype. It's difficult to find a balance between making him an actual person and keeping him Victorian.
6. Dawn - Oh yes. Dawn is the hardest character for me to write for. She's so inconsistently portrayed during her time on the show. We're never shown much about her actual character. She's used primarily as a plot device to forward Buffy's character in S5. After that, she doesn't do very much on her own. As such, it's hard to get a feel for how she would think or what she would think. I found myself making up development for her based on canon events just to give her some background for her thoughts. Though I've found myself avoiding her POV just to keep this to a minimum (Plus, she's not very integral to the story).