What is film noir as a genre?

I would love you to help me write my paper for my film noir class. There are 2 parts of the paper. The first part is to write about either Leave Her to Heaven (1945), Karakter (1997), or Cat People (1942), any topic, whatever you want to talk about you can talk about all 3 of them or just one (4 pages, double spaced). Part number two, is a Retrospective essay: 8 pages. It is the chance to look back over the genre, your choice of films and emphases.
What is film noir as a genre? What features/qualities of a film warrant its inclusion in the genre? Make use of any of the films we have seen this term, in part or whole, and any others you have seen that strike you as relevant.
The films that were used in the semester were:
The Maltese Falcon (dir. John Huston, 1941; U.S.)
Cat People (dir. Jacques Tourneur, 1942; U.S.)
Casablanca (dir., Michael Curtiz, 1943; U.S.)
Leave Her to Heaven (dir. John M. Stahl, 1945; U.S.)
The Big Sleep (dir. Howard Hawks, 1946; U.S.)
The Killers (dir. Robert Siodmak, 1946; U.S.)
Notorious (dir. Alfred Hitchcock, 1946; U.S.)
Panique/Panic (dir. Julien Duvivier, 1946; France)
The Stranger (dir. Orson Welles, 1946; U.S.)
Body and Soul (dir. Robert Rossen, 1947; U.S.)
The T-Men (dir. Anthony Mann, 1947; U.S.)
Force of Evil (dir. Abraham Polonsky, 1948; U.S.)

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