When you describe a character (age, height, weight, hair color, ethnicity, clothing, etc.), you are creating a characterization. The characterization is all the surface information that is know about a person (in addition to the above: schooling, social class, employment, etc.).
But the True Character is defined by the choices the character makes under pressure. For example, you can describe a successful businessman who is happily married, has children who are doing well in school and in sports, and who has a world of opportunity still opening before him. You describe him as the kind of guy any team would want on their side, the man you would choose for a neighbor, the fellow who would be selected as the jury foreman.
But what happens to this man when his world falls apart: one of his children is kidnapped, for example. Will he remain calm, focused, in control? or will he fall apart? make rash decisions? put others in harm’s way to help the one?
It is moments like these that define True Character.
When you are creating the characters for your book, start with plot…know who must do what in the story to give it drama, or humor, or suspense. Then people the book, but don’t create cookie-cutter characters, stereotypes…unless you plan to stand those stereotypes on their heads.
Do the unexpected with your characters. Remember the mother-in-law who lives with the successful businessman? The character you may have thought was just filler, rounding out the background for this stand-up guy who even lets his wife’s mother live with them? What if, as he begins to fall apart, she steps forward to lead the way, providing the strong shoulder and clear thinking required in the circumstances?
By the end of the story, we should know the True Character of each of your cast, and none of the primary characters should remain as they started. All must change. That is story. That is character development.