When I did my second novel, I had four main characters, and I gave them all equal weight in the story. They ended up jostling each other for dominance in the story. I ended up going back to the beginning and picking the person who the story was going to focus on. Every character has their role. I thought tying this into TV shows is a useful way to look at it:
The Star of the Show: Your main character. He’s on the screen most of the time. The story is about him or something he needs to do.
Special Guest Star: That’s your villian. He’s trying to thwart the main character. But the story is still going to be the main character’s.
The Costars or Supporting Cast: Important characters to the story, but the story isn’t about them resolving the problem–they help the main character resolve the problem. They could even be people who get put into danger because something the main character or the villian does.
Guest Cast: These are the minor characters who come into the story. They may come in for two or three scenes for a specific purpose, then disappear. Like coroner in a police show who explains how the victim died to the main character. Her role is done after that because she’s not needed any more.