how 'dark', downbeat and black ...
Film noir is usually taken in the city, at night, and usually takes place in nightclubs, bars, casinos and cheap hotels. Scenes in the film have a lot of shadows, which are sometimes so exaggerated and stylized to look almost comic.
The characters in the film noir are mostly motivated by greed, jealousy or revenge. The main character mostly private detective or police officer, who investigates crimes such as murder, robbery, gambling, prostitution, extortion and so on. The characteristic of the main character is that he and "bad", it is often torn by remorse, and some of its actions (threats, lies) is no different from the criminals prosecuted.