Janice Hardy had a post yesterday about interviewing your characters - she found an interesting list of questions on The Script Lab (of course, you all follow her wonderful blog and thus already knew this, right? :p).
I've never done this myself, but sometimes I find those questions can be a useful jumping-off point. I don't need to know what my MC's favourite colour is (I actually don't know my own, which makes conversations with five-year-olds incredibly difficult, as they can sense it when I lie). But I had never considered the relationship that one of my MCs (Rosie) had with her parents in much depth, and now that I've started to, it seems there may be some interesting stuff there.
My other current MC (concurrent edits are fun . . . ), Becky, is less of a mystery. She's contemporary, close to my own age, went to my university. It's simpler to imagine many of her opinions and experiences, because most people from my background have certain politics, certain ideas. Not all of them, and she is quite different to me on a number of points, but it's easier to piece her together. That said, there is a whole area of her life that I never touch on - although she has a love interest in the novel, there is no mention of her ever dating anyone before. She's almost 30, she probably did. And while it never needs to make it into the novel, I think it would add some depth if I knew whether she was terminally single, voluntarily single, not that bothered about being single, nursing a broken heart since she was 18, or just too lazy to bother. It may give the character an extra dimension - and even as I'm typing this, I can think of a trait she definitely has that her romantic history could highlight.
Character questionnaires are fun :)