Fictional characters tend to fall into predictable patterns and circumstances (also known as stereotypes). Some of these are cultural stereotypes (the star athlete, the cheerleader, the computer geek, the crooked poltician) and some are literary stereotypes (Holden Caulfield, Elmer Gantry, Emma Bovary).
To keep your characters from being flat, empty stereotypes requires the same kind of care and attention that their physical description does. Each human being is unique through his or her inner world: the thoughts, feelings, and responses to circumstances. Genre fiction at its best is a prime example. Most detective novels rely on a standard plot (dead person, clues, suspects, discovery, solution to the mystery). What makes some of these stories great is not only a macabre plot twise. Far more often, it is the inner working of the detective, the inner working of the suspects, and some of the secondary characters as well. Harry Bosch, Dismas Hardy--these are great characters, people we come to know.
We feel intimate with them. We know them. Give your characters an inner life, a truly inner life, and it will add a layer of richness to your work.