How Would You Spend Your Money?

A simple way to get an idea for a character's motivation is to ask them what they want. This is a question that gets to the core of them as people. If they can tell you what they want out of life, you can keep moving them toward that goal. 

One downside to this question is that a lot of people say "money". Too many characters, especially antagonists, have the desire to acquire money, but that isn't really an answer. Money is a means to an end. If a character wants a lot of money, then ask them, "what would you spend it on?" If they can answer that, then you have their real core.