Robert Mitchum is, along with Humphrey Bogart, without a doubt one of my favorite Film Noir actors. That may - or may not - have something to do with both of them having played Phillip Marlowe in The Big Sleep. Of course, as I as stated previously, the original version of that movie, released in 1946 with Humphrey in the role is far superior to the 1978 version, but that is hardly Robert's fault.
One of the classic Noir Films, the teacher said that all of the elements of the genre - or style, if you prefer - are present in Out of the Past. There are plenty of online sources for what these elements are, so I will not bother to list them here.
We were sad to see the class end. However, the instructor hinted there may be more of these in the future. I and most, if not all, of others in the class, were happy to hear about that possibility!