(Filling in for our own Carole Price)
One of my favorite ways of starting a fight is to ask a room full of film noir fans to define film noir.
It gets ugly pretty fast!
Part of the problem is that a huge part of what people think of when they think noir is the mise en scène--the shadows, the constricting framing lines of staircases or window blinds that evoke a feeling of being trapped, drawn inexorably into doom.
Another part of the problem is that many films noir use a specific plot device, the relatively innocent person who makes one mistake, and keeps trying to "fix" that mistake with additional mistakes until they are drawn inexorably into a doomed situation.
You see the theme there--inexorable doom. It's really common. But is it a requirement to be noir? Some argue that it is, and therefore Maltese Falcon isn't noir, because Sam Spade walks away. Similarly, everyone strives for a structural definition of noir that excludes serial killers (with the possible exception of "M", which is about so many things that we can simply grant it exceptional status and ignore it).
I believe there is a single structural requirement for noir, that, when combined with the stylistic hallmarks and one of two or three very specific world views, defines noir:
A protagonist who has socially unacceptable but perfectly relatable desire, and when he or she acts on that desire, his or her life is ruined.
So in center-of-the-genre stories like Double Indemnity, we clearly see a protagonist brought all the way down by his desire. His perfectly understandable desire (if you like Barbara Stanwyck) is socially unacceptable (though nowadays it wouldn't cause anyone to blink). But what about Maltese Falcon? Hasn't Spade solved all his problems? And what is his desire?
In the book, Hammett makes clear that at the end, Spade has lost the only relationship he truly values, that of his secretary. She's the only good person in the whole book, I think. And in Spade's society of mildly crooked cops and wildly crooked cons, acting on principle is simply not done. So his perfectly relatable desire to actually do his job and get justice for his murdered partner, whom he clearly neither especially likes or respects, is unacceptable in Spade's social circle. That's why the story resonates. That's why it feels like noir. The movie ends a scene before the book. In the book, his Gal Friday hates what he did to Miss Wonderly (Brigid O.). The young woman understands him, but she is repelled by him all the same. His desire has cost him everything, and we are empathetic to that desire, even though his society is not.
You can run through this great list of films noir and see if my hypothesis tests out. Notice that this structural requirement excludes Hannibal Lecter, but may not exclude Dexter--you tell me (I haven't read the series, though I adore the actor who played him on TV).
Thanks for listening to my theory. Let me know what you think!
P.S. I forgot to explain the world view thing. Originally, there was a ton of PTSD being expressed in these films, most notably in Act of Violence. Also, asserting that there is a big difference between what we claims we value in our cultural norms versus what we really desire and do was an important world view. And finally, there is the simple, uncomplicated web of doom that drove many a B film. My favorite movies combine them all. Noir novels range much farther afield, in my experience. What do you think?