One of the most common complaints I hear from writers is, “I don’t know how to make my characters seem realistic!” *cue sad music*
These writers have often tried a variety of character development methods. They might have worksheets and spreadsheets and character interviews. They might know their character’s favorite color, most attractive feature, and every moment of their childhood in chronological order. But none of these things create a realistic character.
You might be gasping in terror because you are certain that your character development exercises have helped you, and they probably have. There’s nothing wrong with that type of character development.
The problem arises when personality replaces depth.