Do you control your characters or do they control you?
I've mentioned before that I am not a fan of those crazy exercises where you "interview" your character or "take her out on a date" and so forth. If other writers find them effective, great - whatever works.
But for me, characters exist already and always have, burbling away deep in the primordial stew of Me, little golems knitted of impressions and experiences and a fair measure of the inexplicable. The job is more excavation than creation, and structure and planning just gum up the works. What I need to do is to get the hell out of my own way, and then the characters are free to bob up to the surface.
Actually, I think they wait like the little green Claw-worshiping aliens in Toy Story, longing to be plucked, to be chosen. Characters have a strong lust for life - at least, mine do; once launched, they are utterly fervid until they've been committed to paper. They demand to be fully drawn and will not let me rest if the impressions are incomplete or inaccurate, and that is a good thing because it keeps me in the chair until I get them right.
I had a character in a book I wrote around four years ago (it never sold) who was vindictive and brittle with a gift for moments of acute cruelty. I knew her well: what she wore, how she did her hair, how she spoke to her family. I spun her out through the book with relative ease, and at the end I was surprised to find that something was missing. It wasn't that she was flat or one-dimensional; that isn't my usual problem. She was plenty complex. She was just incomplete.
I had to start a second book, a sequel, to get to the heart of the problem. I was 70,000 words into it before I realized that I was really just conducting an elaborate character inquest, trying to figure out what I'd missed. I finally found it (would take too long to explain, but in short I had her motivations wrong and that left room for growth that was never evident in the first book) and then I set the entire thing aside, no longer interested in the story once I'd solved the character puzzle. (Well, it wasn't much of a story, to be honest.)
So it's not the characters that control me, but the imperative to know them well enough. How well is well enough? Ah, that's a discussion for another day...