I'm a big fan of getting to know my characters really well before I start writing. My favorite method? Questionnaires. Name, age, hair and eye color, height and all that basic stuff. But then I ask them what their favorite childhood memory is. Or what person had the biggest impact on their life. It's more than just knowing the surface things about these people that live in my head. I need to know what makes them tick. Why they do the things they do.
It really helps to build a story with characters that are that well-developed. Even if your reader never knows that your main character had the chicken pox when she was thirteen or that her favorite ice cream is mint chocolate chip, or that the smell of vanilla gives her a tummy ache. But if YOU know these things, that character is that much more alive to you.
And when your characters are alive in your mind, they'll come to life on the page.
What about you? Do you use questionnaires? What do you ask your characters? What other methods do you use to get to know them?