There are three basic cornerstones to character creation: how your character looks, acts, and thinks. A critical part of this last trait is what your character thinks of himself. Those beliefs influence how he reacts to people, and how he interprets their reactions to him. A man who thinks of himself as dull, for example, might assume that the woman who turns her back to him has grown bored. In reality, she’s just simply looking for the bathroom.
Real people—and good characters—almost never see themselves as others do. Knowing this aspect of your character will make them richer, deeper, and easier to write.