E.M. Forster, who coined the terms round and flat characters, said the true test of whether a character is round is whether the character can surprise you in a convincing way. When a round character surprises you, it’s often because the character learned something about himself of herself. Through the character’s unforeseen insight, we learn a lesson about ourselves and others that wasn’t apparent before we started reading the story. Unlike flat characters, round characters reflect a range of human experience. We remember them because we can relate to them and because we can grow vicariously through what they learn about themselves.
Multi-Dimensional Equals Memorable
A flat character demonstrates a single character trait. A flat character might be a grieving widow who refuses to move on after the death of her husband. She has reason to be unhappy, but a flat character such as this isn't memorable if she remains inconsolable forever. It’s often easier to relate to and remember round character because they reflect our own multi-dimensional experiences. Flat characters have their place in stories. But their role is most often to complement a main character, a round character. These central characters are the ones who stick with us long after we’ve finished reading.