Plot and character are the same thing. A story’s actions should arise out of the decisions and reactions those particular characters make. Different characters would drive the story in a different direction. Why are these things happening to these particular people and not someone else? Changing the characters, the kinds of people they are, would change the story. If the events of a story would happen no matter who the characters are, then the characters have no impact on what happens, and why should I want to read about them?
This really made me think about the book my son and I have been working on for about a year now, I am not sure that our characters are unique enough. We have felt for some time that the plot needs work. Maybe this is why,