My biggest tip for fanfic writers is this: if you get a character's mannerisms and speech pattern down, you can make them do pretty much whatever you want and it'll feel in character.
Logic: Characters, just like real people, are mallable. There is typically very little that's so truly, heinously out of character that you absolutely cannot make it work under any circumstance. In addition, most fans are also willing to accept characterization stretches if it makes the fic work. Yeah, we all know the villain and the hero wouldn't cuddle for warmth in canon. But if they did do that, how would they do it?
What counts is often not so much 'would the character do this?' and more 'if the character did do this, how would they do it?' If you get 'how' part right, your readers will probably be willing to buy the rest, because it will still feel like their favourite character. But if it doesn't feel like the character anymore, why are they even reading the fic?
Worry less about whether a character would do something, and more about how they'd sound while doing it.
