Most writing manuals advise you to figure out everything you can about your characters. Know their goals, their ambitions, their secret fears and desires. Know the worst thing that ever happened to them and the best. Interview them about their favorite color, their favorite song, their first pet, their first crush. Delve deeper: know their astrological sign and their Myers-Briggs type. All this, the manuals say, you must know before you start your story if you're going to have your characters act consistently, with integrity, from a deep sense of identity.
I think that's all a waste of time. It's yet another prop to make us think we're working on our writing when we're not. As preparation for writing, we'd be better off cleaning the house.
To me, a character is someone you meet the way you meet a new person in your life. The person catches your eye, makes gestures, talks, acts in some way that makes an impression on you. You're interested; you pay attention. You talk to the person, go for a walk with him, watch him play with his little sister. Gradually, you get to know him. Soon you're good friends. You understand a lot of things about each other without the two of you ever explicitly hammering them out. At the same time you respect the fact that some parts of his life aren't part of the story you share with him. You're not too concerned that he reveals only what's relevant. Sometimes things he does or says still surprise you.
I'm exaggerating, of course. But the truth is, character is revealed through action and dialogue. What I'm suggesting is that the character reveals himself this way not only to the reader but also to the author. I know a few things about my characters - age, name, number of siblings, where they live, and a few things about what they like. Then I put them in a situation and see what they do. By what they do I get to know them better. I get to know them while I'm writing, instead of while I'm preparing to maybe someday start writing.
If something my characters do or feel or say doesn't make sense, I dig in and question them to find out why they behave that way. Why does Daphne feel sick to her stomach each time a math problem looks hard? Why does Ivan always want to turn aside praise? When they do something unexpected, I learn a little more about them. Still, I don't know Ivan's or Daphne's favorite color. I won't know unless it comes up as part of the story. And then they'll tell me.
You could say I share information about my characters with myself only on a need-to-know basis. To me, that makes the task of writing a lot more engaging. If, godlike, I knew everything about my little minions, then writing the story would hold no surprises for me. Before they acted, I'd have to check my mental checklist of all their traits and preferences, and that would determine their response. How boring!
Instead, like my readers, I watch closely to see what my characters will do. Sometimes they surprise me. For example, I never knew Bran was going to leap through the mirror with Ivan and Daphne into the Land of Night until the moment he did it. With that action the big, clumsy and good-hearted mathematician became a companion, a comrade, an important person in the story with a mind and hopes of his own. Through his actions he became a favorite of mine, of the artist's and of many readers. I would never have planned it that way.