As a beginner at writing fiction, I've been reading to acquire some background about the art and craft of novel writing. As a result, I have a question which relates especially to my chosen genre, although I somehow wonder whether it's necessarily genre-specific. In the murder mysteries that I enjoy reading, the detective (main character, protagonist, whatever) doesn't usually arc: Holmes, Poirot, Morse, Smiley, Dalziel — the list goes on. Yet many of the craft books I have been reading seem obsessed with the notion that unless the Main Character has a character arc (with the result that readers see them change at the end), the story will be in some way incomplete or even second rate. So my innocent question is: does a detective MC have to arc in today's murder mystery novel?