Complexity to the point of incomprehensibility is one of the hallmarks of film noir. This would be a problem in an ordinary mystery story, where the solution to the whodunnit is the whole point. But even though noir stories often involve mysteries and detectives, the whodunnit aspect is never important in a true noir.

Most film noir movies don't use the flashback/voice-over technique of storytelling, but this technique has become closely associated with film noir for two main reasons.

One is that a few of the most iconic noirs do make use of it, so people who have only seen a handful of the true classics are more likely to associate this technique with noir. The other, and I've never seen this commented on elsewhere, is that the use of a voice-over closely replicates the tone and feeling of the novels on which a lot of noir movies were based.

The femme fatale is more glamorous, but she's not the only feminine archetype in the world of film noir. The other side of the femme fatale is her opposite number, the “good woman” who just can't help but love the doomed protagonist.

The femme fatale is second only to the hardboiled private eye as a noir archetype. The origins of the private eye archetype are in the pulp magazines from which film noir drew a lot of stories, but the femme fatale goes all the way back to Helen of Troy.

A femme fatale is a dangerous woman, a woman who uses her charm and her beauty to get what she wants- often by manipulating and duping a private eye. This is the theme of the very first noir of all, “The Maltese Falcon,” but Bogart as Sam Spade turns the tables on femme fatale Brigid O'Shaughnesy in the end. This is a typical development for a lot of noir movies, although sometimes it ends very badly for the detective as well, as in “Out Of The Past.”