First of all, character voice is important. Character voice sometimes comes naturally, other times it must be practised - but the thing is you can master it. The most important thing is to watch episodes, listening carefully for the flow, the certain distinct way of wording things that every character has. Make sure you're watching from a period that you're writing in - Wes in season three of Buffy speaks slightly differently than Wes in season three of Angel. Develop an ear for listening not to what they're saying, but how they're saying it, how it feels. Transcripts can help as well, in wording and slang - try the ones offered at Buffy World.

Character voice is simpler than actual characterization, in my opinion. Regardless of how "Dawn" Dawn sounds while killing Lilah, she is still out of character. It's harder to discern what exactly a character will or won't do. Character voice can be learned from a few episodes, characterization takes many more episodes. You need multiple examples of how a character acts and reacts under diverse circumstances. You also need a clear grasp of the character's history as displayed in canon, and an understanding of the things that make them tick.

In this, transcripts are invaluable. For those episodes you missed, those episodes you can't remember clearly, ect. - the transcript is a must-read. Even if you watched the episode and think you remember it well, still read transcripts. You'll find things maybe you didn't notice before because you weren't looking for them. Little details that make all the difference to your fic, but you didn't consider important enough to remember, can be found in transcripts.

A word on unrealistic situations - the most important thing is to think. No, really. You have to use your brain. Does your fic make sense in the context it's presented in? If it takes place in an alternate universe, does the fic follow the rules you established for that universe? Also, something important - if you need a long author's note to explain things in the fic, in my opinion you've failed. Many sites, including this one, do not put author's notes on their copy of the fic. If your fic does not make sense without the crutch of the author's note, then you need to rewrite your fic so it does. I know this seems a little harsh, but trust me - it's good advice.

Another thing about realistic versus unrealistic situations - do your research. If someone has a medical ailment, learn about the ailment, and enough medical terminology that any doctor who happens to be reading it wouldn't go - "hey, that's not what that disease is!" That's just an example - the general idea is that if you're writing about it, make sure you know enough about it to be writing about it.

Yes, I know this seems like a lot of work - it is. Writing good fic is a lot of work - but it's rewarding, and believe it or not, it is fun. There's nothing quite like the feeling you get after writing something you know is good, and then finding out other people agree. But is there any way to write a good fic without doing piles and piles of research? Of course there is! Some fics don't need any research at all - however, it's always your best bet to do more reseach than you need to - the outcome will be better than if you do less than you need to. However, it's probably a bad idea to try to stuff all your research into the fic - use just what you need to get your point across and be realistic.

Do your research - on characters, on situations, on whatever is applicable to your fic - and make sure everything makes sense. You do those two things, double-check things like spelling and grammar which I consider pretty much common sense, and your fic should be readable. I'd suggest getting a good beta reader - it may take some time before you find the right person, but once you do, you won't know how you ever wrote without one. A beta reader not only acts as editor, but they can offer an invaluable second opinion on such crucial things as characterization, and story plausibility.

So get writing! I look forward to reading whatever you come up with!