Little habits and speech patterns are important for building believable characters. These can be pensive, humorous, annoying, physical, or simply endearing. Everyone has them, whether or not they are conscious of them.
When I mention pensive I’m referring to the expressions and gestures a character might make when he or she is thinking or considering something. Humorous, think of characters like Inspector Clouseau or Maxwell Smart. Annoying should be easy. If you don’t know anyone who has any annoying habits, you are either too easy-going or not paying attention. Endearing should be easy, too. Think of your favorite aunt or grandparent.
These are another element that provides depth and a realism to your characters.