Characterization is the portrayal of those essential traits which form the unique, distinctive personality of an individual human being.
Characterization requires an extreme degree of selectivity. A human being is the most complex entity on earth; a writer’s task is to select the essentials out of that enormous complexity, then proceed to create an individual figure, endowing it with all the appropriate details down to the telling small touches needed to give it full reality. That figure has to be an abstraction, yet look like a concrete; it has to have the universality of an abstraction and, simultaneously, the unrepeatable uniqueness of a person.
In real life, we have only two sources of information about the character of the people around us: we judge them by what they do and by what they say (particularly the first). Similarly, characterization in a novel can be achieved only by two major means: action and dialogue. Descriptive passages dealing with a character’s appearance, manner, etc. can contribute to a characterization; so can introspective passages dealing with a character’s thoughts and feelings; so can the comments of other characters. But all these are merely auxiliary means, which are of no value without the two pillars: action and dialogue. To re-create the reality of a character, one must show what he does and what he says.