I know I've written about character development before. It's no accident that I'm showing you another way to look at it. The creation of rich, multi-dimensional characters is key to developing a mature novel.
To give your readers believable, full-bodied characters, it's good to look at them and develop them from different angles. So, here are two more factors to consider during characterization:
1. Develop individual actions and reactions for your characters. We all have mannerisms that are ours alone. They make us unique. Your characters need them too. Don't overload your characters with unusual actions; however, or they may become caricatures. Give them just enough unique mannerisms to set them apart from the other characters.
Through the development of unique actions and reactions for your characters, your readers will learn what to expect from them. After you have established a pattern of behavior and reaction for our characters, you can use this knowledge to increase the tension within your story. Lead a character toward an event where their predictable reaction would end in disaster. The readers will have the delight of biting their nails to see if your character reacts predictably and what happens as a consequence.
2. Another way to develop multi-layered characters is to pair the actions of your characters with their physical attributes. For example, perhaps your main character is very precise in his actions and quite methodical and deliberate. His clothes reinforce this perception by being fresh and his grooming is impeccable.
If you build a character such as this, your readers would naturally expect the character's apartment to be tidy and organized. What does it do to the picture if the person lives in a mess with dirty dishes in every room, newspapers and piles of books strewn about, etc.?
Knowing this adds another layer to that character and gives the readers a bit of information that lets them believe that this person is not quite as predictable as they appear. Without specifically telling them, your readers have been alerted that this character might indeed delight them with some aberrant behavior later in the book.
Look at your characters from many angles and you'll develop much more interesting characters.