This morning I'm thinking about how I come to know my characters. They're rather like 'real' people I come to know gradually. Only very, very slowly do I understand why they act as they do. Others have written about various strategies, but the whole issue of world-building and understanding motivation seems very murky to me.
I've tried the list method, plumbing deep into surface description, their wrinkles and warts, the color of their eyes, what they might like to eat for breakfast or, for heaven's sake, never touch at all. Identifying their secret nightmares and phobias (snakes?) is more fun. Their irrational acts or 'mistakes' or quick tempers. What would cause them to curse or fight or run away entirely.
Sometimes I'll find a photo of someone and recognize that person as one of my characters, somehow loosed on the world. Or I'll have one character fantasize about another and dream them into being.
But it is through action that I learn most about who my characters are, through what they choose to do. For it is in the moment of conflict that we are unmasked and our own character revealed.