Character charts, in which the writer is prompted to answer specific questions about physical, historical and personal character traits, can help spur deep character development. These charts are simple to use, though they may take some time and thought, and have the added benefit of creating a great reference later when you can't remember what color your hero's eyes are. However, you can get caught up in character chart creation and forget to write your story. Use them, but limit how deep you go.
Pretend you are a reporter preparing to write an article on your character. Put together a series of questions or alternatively look up a series of questions online you can ask your character. Set aside some time and either write out the answers to the questions or record spoken answers, depending on what makes your creativity flow better. Use these answers to build your character.
Sometimes character development stops dead because writers forget the many, many ways to show who a character is. Physical description is important, as a large muscular man may deal with things very differently from a little girl. The character's personality, actions and dialog can be developed either in-story or by writing little vignettes outside the main story for the sole purpose of getting to know a character better. The most-neglected characterization method is describing a character's surroundings either in or out of story. Any naturalist will tell you that an animal's habitat, its way of making a home and its way of "clothing" itself are all critical to an understanding of the animal. Your illiterate character who owns shelves of books or your intellectual character who dresses like a streetwalker so people won't look below the surface may be best characterized by the conflict between their surroundings and their true natures.
According to writer and publisher Debra Dixon, characters need goal, motivation and conflict in order to be dramatic enough for a story. The character must have a goal that is not easily achieved but that the character wants more than anything in the world. His motivation to achieve this goal must be logical and rooted in his personality. And there must be conflict, seemingly insurmountable obstacles to his achieving that goal. There is much more to the GMC method, but simply writing out these three aspects of each character and putting the characters at odds by interlinking them will go a long way toward deepening your characters, making them compelling and driving story plot. The simplest example might be a novel about racing, in which each character has a different motivation but the same goal -- to win the race. In this example, each character must overcome every other character to achieve the goal.
The 5-Why system was developed to help engineers get to the root of a problem, but it adapts readily to character development. This simple process requires that you ask "Why?" about a problem, then the answer, then that answer, through five iterations of "Why?" For instance: My character is afraid of spiders. Why? She's afraid of their long legs and bites. Why? Because her brother used to drop spiders on her when they were children. Why? She believes he was jealous of her. Why? Because her father loved her more. Why? Because a previous daughter had died, and he saw her as the replacement for that baby. As you can see, you can get a lot of character depth out of the process. Do be careful to keep every answer centered on your character, however, or you will find 5-Why leading off on a tangent.