I meant to post about this last week, but I haven't gotten around to it yet.  However, one thing I noticed in the stories we have been looking at is the use of flashback.  These tales have a tendency to be told as the protagonist recalls the past.  In "Double Indemnity," Walter Neff relates the whole film by telling Keyes the story through a recorder.  Then we can look at The Postman Always Rings Twice, where the reader finds out at the end of the novel that it is a confession before Frank gets executed.  And then lastly with "Out of the Past," a solid chunk of the film is Jeff recalling his past life.

Why do this?  I don't know if that's a common characteristic of noir, but it is definitely present in these works.  Is it because people are nostalgic of the past and therefore want their entertainment to be told to them in recollection form?  I don't have an answer, but I certainly would like to hear possible theories as to why these works use this technique.