Writer Walter Mosley has some good comments at Time.com, among them:
With the original hardboiled detectives, there was an existentialism that entered the genre in the '30s and '40s. There was no connection to the world. No mother, no father, no sister, no brother, no friends, no dog, no regular apartment. If you get arrested, they throw you in jail and you can stay there because you don't have any responsibility outside of the case.Mosley has identified the problem not so much with crime as crime series. If a character reappears over several titles, should he change? That's the real problem for an author lucky enough to hit on a successful formula.
With a person like that, there can't be character development, so you actually give up one of the most important aspects of the novel. And that's problematic. The onus now is, How do I create character while also moving forward the mystery, the plot, the crime, the resolution?