When we set out to write a story, we know which characters are the main characters, the ones readers will cheer and invest emotions in. That is until books are turned into movies and actors cast to play supporting characters do such a tremendous job, they steal the show from main characters.
Did you know the main characters in Pirates of the Caribbean were Elizabeth Swan and Will Turner? Jack Sparrow was a supporting character . . . until he stole the show.
Did you know Phil Coulson was only a supporting character in The Avengers. Writers thought it was okay to kill him off . . . until fans rattled their cage to have him resurrected.
The same happened in Thor: The Dark World. They killed Loki, then realised he was too big a character to knock off, and they had to bring him back. He was supporting Thor, but we know how that went down with Loki fans.