Managed to dig up some old film studies lecture notes on film noir that I studied, which essentially explains the characteristics of the genre.
Film noir is a term used to describe crime drama movies from Hollywood that are often focused on sex, crime, and corruption.
Film noir movies were mostly made from the early 1940s to the late 1950s in the United States, and they were usually filmed in black-and-white. The term "film noir" comes from the French term for "black film." Film noir movies include many different genres of movies, such as gangster movies, police movies, and detective movies.
Film noir movies were often filmed so that there were many dark shadows in the movie, even on characters' faces. The Hollywood film noir movies were influenced by German film directors such as Fritz Lang, who used dramatic lighting techniques. Another influence on film noir movies was 1930s French books or movies about heroes who would die at the end of the story or stories with sad endings. Film noir movies were also influenced by crime novels, such as the detective and crime stories by Dashiell Hammett, James M. Cain, and Raymond Chandler.
Film noir movies are often shot in cities at night, and the stories often take place in bars, nightclubs, gambling clubs, casinos, and cheap hotels. This makes film noir movies look very dark, because the scenes are shot at night, or inside dark places like a bar or hotel.
Film noir movie scenes often have a lot of shadows, such as the shadows of window frames, blinds, or stair banisters. There are also often dark shadows on the movie scenes, even on the faces of the actors. All of the shadows and darkness make the film noir movies feel mysterious and stressful.
The characters in film noir movies are often motivated by greed, jealousy, or revenge. The characters in film noir movies are often private detectives or police officers who are investigating crimes committed by bad criminals and gang members such as murder, gambling or prostitution. In film noir movies, there are often stories about robberies, heists, extortion of money (which is called "blackmail"), or murder.
In film noir movies, even the heroes or lead characters are often bad or partly bad in some way. For example, the hero of a film noir movie may help an innocent man or woman who is being threatened by a criminal, or rescue a woman from a criminal gang. Yet the hero or lead character may also do bad things, such as commit crimes, hurt or threaten people, or tell lies.
Often, the hero or lead character of a film noir film will be bitter, sad, lonely, or depressed, because of bad things that happened to them. The person watching the movie has to decide if they think that a hero who does both good and bad things is a good person or not. The stories from film noir movies can make the audience think about how people should act, or how people should make choices in their life. This is one of the things that people like about watching film noir movies.
Film noir movies are not happy movies. In happy movies, like romance movies, people fall in love, get married, and live happy lives. In film noir movies, sad things happen. A good man with a wife might need money for their rent, and borrow the money from a criminal gang leader, because the good man has no other way of getting the money. Then the bad gang leader might force the good man to do bad things to pay back the money. Maybe the bad gang leader will threaten to hurt the good man's wife. Then the good man has to decide between protecting his wife or doing a bad crime.
People who watch film noir movies like to see stories about people who have to make hard choices, or people whose lives have sad things happen in them. Real life is not all happy and perfect like in romantic films; people often have to make hard choices or have sad things happen. For this reason, film noir movies are a bit more like real life than romantic movies.