Buckle up, friends. This is a long one. Mostly to help me remember/collect some tools I’ve found.
I find these tools for building character much more useful than things like “the character interview” — where you ask things about favorite colors or food or whatever — because really, who cares? Those things are just trivia. Just like your knowing that my favorite color is green or my favorite food is scallion pancakes* doesn’t mean that you actually understand anything about me.
Pieces of trivia don’t reveal character motivation and drive and desire and limitation,** which are the things you need to consider when figuring out what a character’s Problem is and what they’re Going to Do About It.