You've probably heard the advice to ask your characters questions. You know, what is your name? What is your quest? What is your favorite color? What I've tended to notice about when I do those, is that sometimes my characters are really open with me. Sometimes, they shy away from a question, or I can tell they're holding back.
So how do you get around these difficult characters? While I was at Writing for Charity, Jennifer Nielson talked about the psychology of character. She worked at a camp for troubled teens, and for them, they had to do fill-in-the-blank statements. So, for example there might be a question like:
I'm happiest when _________
I trust ________
I'm afraid of __________
It's amazing what your characters will tell you with this format that they won't with the interview. Also, you tend to notice patterns crop up. One of my characters feels a longing for love but fears it at the same time. And I never knew. So next time you have a character giving you a hard time, one you want to know better, or just for kicks, come up with these types of questions and see what you get!