There are a lot of character worksheets floating around. You might have seen a few yourself. They list hair color, eye color, occupation, age, clothing preferences, voice qualities, car, hobbies -- the lists vary, but the idea is the same. Fill out this list, and you will have a quick reference sheet to help you recall whether the police detective drives a Ford or a Buick and what color the pretty waitress's eyes were in chapter three. It's a good and useful aid to memory, and any good copy editor will have several such forms on hand to help them do their jobs.
But if you, the author, fill out a sheet like this and think you've created a character, you might have only done half the job. Sure, you have to remember whether the heroine's house is a ranch or a Cape Cod. But if I tell you, "Juliet lives in a white brick Georgian house," do you understand her character any better? Not really. Not without knowing why she lives there. Is it her dream house, or does she think it's a lemon? Did she inherit it? Did she buy it in a rush when her job relocated her across the country? Her relationship to the house -- the why of the house -- tells us more about the character than the fact of the house alone.
When you fill out one of these worksheets, ask "Why" at every stage. Sometimes the answers might be a little pat. Why is the romance heroine 27 years old? Because this is a good age to marry and start a family. It might really be that simple. Then again, think about how your story would change if your romance heroine was 67 or 17. Maybe now she doesn't have to worry about getting pregnant. Or maybe the hero has to worry about the age of consent. How does a simple detail like age affect the essentials of the story? Whatever the effect, understanding that will help you understand the "why" of your character choices. And when you understand that, then you understand your characters in a deeper, more meaningful way.