There are lots of things you can ask yourself when creating three-dimensional characters for your novel or screenplay, but one of the best was suggested in a New York Times essay on marriage:
We seem normal only to those who don’t know us very well. In a wiser, more self-aware society than our own, a standard question on any early dinner date would be: “And how are you crazy?”
The article, titled, Why You Will Marry the Wrong Person, by Alain de Botton, is well worth a read, but I mention it here because "How is your character crazy?" is an excellent question that will help you create a character who comes alive on the page.
What kind of craziness? Botton gives these examples:
Perhaps we have a latent tendency to get furious when someone disagrees with us or can relax only when we are working; perhaps we’re tricky about intimacy after sex or clam up in response to humiliation. Nobody’s perfect.
You can warm up by thinking about how the people in your personal life are crazy--what imperfections set them apart? And how are those imperfections occasionally beneficial rather than harmful?
If you like living dangerously, ask a couple of people to whom you are close how YOU are crazy...but only if you're sure the relationship will survive their obviously mistaken impressions of you.