Make the Character as In-Character as possible
Most authors apologize for making the character "OOC", the main reason why that happens is because they don't know the character.
Don't make a fanfiction on a character if you can't grasp their persona. The only time it's okay if it's a side that hasn't been shown or if it's a minor character that only had 1 line.
If you have an OC who's entirely sass and kicks everyones ass, don't suddenly make them all shy and meek [unless they have a multi-personality disorder]. Even though you made the character it doesn't mean you can change them just because it doesn't fit anymore.
If the character had a heart-wrenching past and doesn't care, they won't suddenly break down crying seeking comfort in whoever. They clearly don't care so leave it at that.
If it hasn't happened in the show/movie/book/manga/comic/etc. then it won't happen until the author says it does.
Stick to the characters personality, not how you want it to be.
