I had this professor in college... Let's just say she wasn't my favorite. She wore skirts that were unprofessionally short. She called a quiz a quizzy-poo. She once gave me an answer to a question that I looked up later and found out was woefully wrong. I couldn't believe that she was my professor and didn't want to pay attention in class, because to me me was just a professor who needed to grow up a little.
Sometimes when we first meet our characters, they strike us as something like this. They might be painfully shy, or the classic cool kid, or maybe your villain wants power really, really badly.
One day, I was surfing Facebook in the middle of said professor's class. I had only come for the quizzy-poo at the beginning anyway, and it would be rude to leave right after, right? I scrolled down the page, my mind feeling a little bit like that white noise on an old TV set. Then a picture flew past my eyes. Something about it made me stop, and focus again. I pushed that up button so that the image was zoomed in, and glanced between it and the professor in front of me. It was the same person! My professor's smiling face stared at me from the screen, her arm wrapped around the shoulders of a good looking man who must have been her husband. The caption plead for people to share the image, because they were trying to adopt a child from some group. I never did figure out how sharing the image would help them adopt a child, but when I looked at the professor standing at the front of the class, she was suddenly a different person. She wasn't my ditsy professor anymore, though she still had all of those qualities, but I could see through the outer her, to the woman who couldn't have children for whatever reason, and wanted to adopt one of her own.
When we first look at and write our characters, they seem so one dimensional, and no one wants to be around someone like that. Your readers will pick up your book, read the first two chapters so they can say that they tried, and then (maybe) if they're polite (meaning, if they're family/friends) they'll finish reading because it would be rude to get up and leave in the middle of it. More likely, though, they'll check out as soon as possible.
So before you say you think you know who your character is, think about their other dynamics. That super shy girl might love to do karaoke. The jock might also play the saxophone. Your villain probably has good intentions. So what are they?