What is the author's style in Double Indemnity by James M. Cain?

There is one significant disadvantage, as far as a novelist is concerned, to telling a story from the first person viewpoint--the writer cannot get into the minds of any of the other characters, except the protagonist. Therefore, all the other characters have to be portrayed solely through the voice of the main character. Cain chooses to portray the other characters mainly through their dialogue. He does not allow Walter to think very much about the inner lives of the other characters for a very good reason. Walter is a self-involved person who looks upon others either as people to whom he can sell insurance or, as in the case of women, a source of sexual pleasure.