Designing backstories for the characters that go beyond what is likely to be mentioned outright in the story. I've heard some people say that writing additional backstory is a waste of time, but I think that when you want to write characters that really feel like living people, it's easiest to do this when you know the details of what makes them who they are, what their lives are like outside of the snap shots you get to see. Etc.
Example: I know quite a lot about the lifestyles of Mrs. Moon's friends Jun and Daphne, their children, marriages/lack thereof, and so on. And how these things effect or were effected by their views of the world.