Authors use characters to show the theme. The story is the outgrowth of the theme. If an author wants to write about love lasting forever, he may write a love story with two people who realize that they are going to die and find the fountain of youth, or something like that.
I think that characterization it's the most important thing in literature. Every autor use characters to interact with reader. Characters reveal the main idea of the story. They can symbolize different problems or they can motivate you to do something.
