Whenever I'm in the middle of story I experience an interesting phenomenon. My characters take over my life. I could be driving down the street and I suddenly hear my protagonist railing against people who drive too close to your back bumper. Or standing in line at the grocery store, my antagonist whispers in my ear to move to the speed line because no one would notice I'm one over the item limit. Before you think I suffer from schizophrenia, I know these people aren't really there. But they've found a corner of my mind where they hang out, waiting to surface with a detail or even a twist for my story.
Characters are real to those who write them. And they have to be real to those who read them, too. That means knowing everything you can possibly know about them. I know what Claire had for breakfast, how her favorite sweater feels, that she goes to the dentist regularly, and that in sixth grade she had an apple with her lunch every single day.
All of these details are important to help me understand who she is and how she would behave in any given situation. Some of this knowledge will appear in the story directly, but most won't. And that's every important to differentiate. Readers don't want to slog through a list of elementary school teachers your character had starting in kindergarten. They want the action. But how that action unfolds is determined by how much you know about your character.
When you chose a character for your story, you thought about age, sex, appearance, desires, loves and hates. But the everyday life or your character is what's going to help you make a real person of her. So take her with you on your day. When you drive to work think about how your character drives - is she a solid driver or subject to road rage? When you order lunch, think about where your character would sit - by the window to watch people or with her back to a wall? When you sit down after dinner, argue with your character over which TV shows to watch. The more time you spend with her, the better you'll know her strengths and, even more importantly, her flaws.
Plus, you never know who'll show up for dinner!