Do you know your character's favorite color?
This question was brought on by that common (perhaps cliche) remark in novels and movies alike. The "but I don't even know your favorite color."
It's the moment we realize that our heroine (or hero) is finally admitting to herself that she is falling in love. It usually comes after she's been through a life-changing experience with the hero, and while the audience has been secretly willing for her to fall in love with him, she is only just coming to that conclusion on her own.
She wants to know him.
I think it can be easy to do that with our characters too. Writing a story is like going through that life-changing experience with our characters. But we need to hit that point in the story where we say, "but I don't even know your favorite color." Because once you dig deeper into who your character is, and know things about them that your readers never even see, they will become so familiar to you that you'll be able to see why they resist you at every turn.
Think about the people you know best. You can answer questions they haven't even asked yet. You can predict things about them, or buy them that perfect gift. Why? Not because you know their favorite color, but because you love them enough to take the time to be familiar. Each of your characters deserves that, and your story will be richer because of it.