Often responding to Philip or Sam, the private investigator (PI) may be identified by his coat and hat. His natural habitat: the wet street corner or, unauthorised, another personâ€™s home. He is commonly accused of committing the very crime under his investigation. You will find him lit starkly, from the side. He is good at getting women into bed, but they often turn out to be malevolent villainesses. He is American.
The PIâ€™s bloodlines flow deeply into the tradition of masculine heroes. His characteristics loom so large over Western popular culture that it can be hard to make him out. This is the problem facing any book on the film noir detective: being a chap, in a movie, trying to solve a problem, he is as inscrutably general a cultural trope as the femme fatale. What makes a PI a PI, and not just some other kind of leading man? You canâ€™t even really chalk him up to an era, since he has existed since the early days of film. …
[The] famous five film noir traitsâ€”oneiric, strange, erotic, ambivalent, and cruelâ€”were neither clear cut nor all strictly necessary in order for a film to be noir. This genre is yoked together by a general ambienceâ€”an aura of darknessâ€”rather than any true collective character. If the film noir is about one particular thing, Iâ€™d say it was about bad people. It is therefore about crime, and the investigators of those crimes. Enter the PI.
Hat tip to Karen L. Myers.