Sometimes the best place to learn more about your characters is from your characters themselves.  It is important, however, to make sure you have a concrete launching point before you do this.  You can’t just say create a character with brown hair and call him Bob and expect this method to work.

Your mission, should you choose to accept it, is to jump into one of your story projects and hold a conversation with a character other than your main character. It does not have to be in good proper form, it just has to be a conversation that helps you grasp the character better.

Example: Thief of the Crown - Me vs. Topper

A: Excuse me, I'm looking for a book.
T: How hard are you trying? I am assuming you have at least discovered you're in a library?
A: Uh, yes, I was aware of that. I'm looking for a specific book, actually. I was told to come ask you about where to find it.
T: I do not work in the library. I am a scholar.
A: Well, I was not told what you do for a living, but I was told you could help me.
T: Who told you this?
A: Rowan.
T: I know no Rowan. You must be mistaken, and therefore I feel no need to help you.
A: But you will anyway, I'm sure.
T: You may hold your false expectations if it will humor you.
A: Topper, please, I need a book on deception and secrecy during the final dynasty. I need a specialist to help me with my research.
T: Are you implying that I am a specialist in deception and secrecy?
A: I am implying that I know the university's tuition has increased because they take a 2% net loss on every investment and fundraiser they hold, and I know that you are the one in charge of the money reports, and also the one that people tend to trust with monetary issues.
T: These are all true statements.
A: And I know that the University is in fact making 110% on average. Oddly enough, 12% always seems to disappear between your fingers.
T: Are you accusing me of embezzlement?
A: I am asking a professional to help me find my book. Or just babble off to me everything he knows.
T: You think you are clever.
A: I can be. You going to help me? Or do you want me to accuse you of embezzlement?
T: Cute.  You can either post them here to this post, create your own post, or just do it for yourself personally (or not do it at all. There's no requirement). I just thought some kind of involvement would be fun.

What did I learn from this exercise?  Well, I don’t think I knew how dishonest Topper was.  You know your story and you know your characters completely; that information gives you power to ask the hard questions that no one around them is going to ask.  In the process, you will understand your characters better and maybe even find a hidden motivation you did not expect.