The actual definition of what makes noir noir is a bit undetermined. But you’ll know it when you feel it. It involves an undercurrent of sensuality and sexuality (not the same thing), moral ambiguity (a heroic character does not necessarily have good values or make the societally correct choices), and a dark visual style (not necessarily black and white, but it does help, and all my choices are such).

The Maltese Falcon. This is widely considered the first film noir. In probably his most famous role besides Casablanca, Humphrey Bogart plays detective Sam Spade. The movie is better than the book, although the dialogue is copied almost line-for-line, because it’s hard to convey in prose what these characters are feeling. You can talk about it, but you can’t give inflection on paper, and it’s hard enough to understand even when watching them move, and talk, and interact in a hundred subtle ways.
Remember context. As Ebert notes, in that time and place “Hard men, in a hard season, in a society emerging from Depression and heading for war, are motivated by greed and capable of murder. For an hourly fee, Sam Spade will negotiate this terrain.”
Sam Spade is outside of, and in many ways above, the scheming going on around him. The important thing is not just that Spade goes against the world and defies the standards of those around him, but that he keeps his own code. So he is not moral, and yet is.
The Big Sleep. Philip Marlowe is also played by Humphrey Bogart. Flirtation. Intrigue. Flirtatious intrigue. Intriguing flirtation. There’s no way to ’sum up’ this movie concisely, and a non-concise version would give too much away. I leave it to you to discover.
Random Trivia: the name Brody, which comes from a character in this movie, was used with varying connections on the TV show Charmed. (Agent Brody is used as a flirtation and then love interest, is mistaken for a killer, represents moral ambiguity, travels into an alternate reality* which is a Maltese Falcon homage where he plays a Spade/Marlowe-type character, and is eventually killed.) The multiple levels it works on are even more amusing if you know where he’s coming from.
The Third Man. Orson Wells’ absence is almost as conspicuous as his presence, and though the picture is not incredibly hard to follow or figure out (as some of my readers may find The Maltese Falcon or The Big Sleep to be), the motivations of the various characters are complex and compelling, as are their exchanges with each other.
Notorious. I’ve already written praise for this movie in various other posts, and it will likely come up again before the month is out. It’s fascinating how many genres the movie covers. it’s romantic, suspenseful, set around wartime intrigues, but it’s pure noir.** Moral ambiguity is more subtle in this movie, which doesn’t explicitly address many of the true stakes, but doesn’t let you forget them either.
Shadow of a Doubt. Like The Third Man we have Joseph Cotten, and like Notorious we have Hitchcock. Is he or isn’t he? You tell me.
Double Indemnity. Based on a True Crime (whose perpetrators were apparently nowhere near as quick-witted as their fictional counterparts) the movie starts simple and becomes more and more involved, as will you. Brutality, attraction, suspicion, great music and cinematography.

The stuff noirs are made of.
* Odd as this scenario may seem, it works quite well. The great benefits of shows that have an ‘unreal’ premise to begin with is that you can get away with stuff like this without resorting to dream sequences or compromising the feel of the show. Alternate reality? Sure. Time travel? Easy. Bringing together dwarves and avatars and ghosts? An out-of-the-ordinary meeting, sure, but not so out of the ordinary you need to reall take notice. Becoming a werewolf, banshee, Greek goddess, book character, or mermaid? Yes to all. It’s quite fun.
Yes I’m rabbit-trailing, but that’s why it’s in a footnote. (Or whatever you call these things when they’re on blogs.) Unless you’re one of those people who actually read the corresponding notes as you go. In which case . . . may as well read the other one while you’re down here, then go back up and finish the list.
** One could argue caper - though not in a comedic way many think of caper - with the crossing and potential double crossing (like To Catch a Thief, Hitchcock’s heroes stay true, but often it’s probable they won’t and shouldn’t).