Sometime a story gets started with the concept of a really awesome character. When this happens what unfolds around them is secondary to who they are. You know this person well. They live on every page.
Sometimes the character is who gets cast in the conflict you have created. When this happens, sometimes it’s necesary to go back and make sure that these people in your story have dimension, and are right for the roll. No matter how your story develops, you should know your character intimately, the way you know your sister, your college roomate or your spouse. You should know what is going to make them angry before it comes up. You should know why they can’t stand chocolate cake, if they’re prone to fads, if they go weeks without shaving their legs, if they pretend they don’t mind going to Applebee because they know you have a crush on the waiter there, but secretly hate it.
Here’s my quick list, the one I run down when I’m not sure how well I know a character who is important in my story. Obviously these don’t apply to every kind of story, but they can be adjusted accordingly and answered loosely. For example, for “The Fate of a Princess” we are working in a fantasy feudal landscape. Melandria’s favorite band wouldn’t be a good question, but she would prefer the biggest, grandest music available to her and employs an entire orchestra to play at her wedding.
All this info doesn’t have to appear in your book. Most of it won’t be relevant to your story. But it will be relevant to your character’s personality and history. Go through and see if you can answer them all for the character you are working with now.