One of my favourite techniques to get to the bottom of what makes my characters tick is Twenty Questions. All it takes is a blank sheet of paper and one of your character's traits. You then try and come up with twenty reasons "why" they are that way and "what else". As always, when you start something like this you write down the predictable, but by the time you get closer to number twenty you come up with all sorts of unpredictable things. I find this exercise triggers the creative part of my brain and unlocks things about my characters I never knew. I then go back and layer that into the story.
Wish me luck with the layering.