This is one of the few opportunities in our profession where we can take our imaginations to the furthest extremes with no repentance. Do whatever it takes for you to know your character better than he would know himself. Character development is not thaumaturgy, but its final results can be magical. I view it as something we do when we first meet someone. We first notice how people look.
Appearance is everything.
Also, when we actually begin to write, we're going to join our characters at a critical point in their lives. So, I grab a pair of scissors, some glue, an artists sketch pad, and a stack of magazines. I literally cut and paste body parts from the many different figures that grace our household magazines. I'll work this way until I have the person I've imagined in my mind staring back at me. Admittedly, it's not a pretty process, but it works.