No Positive Hero
Instead of a tale about an epic hero like Achilles or Odysseus, noir fiction focuses on characters who are desperate, trapped or evil. Typical noir protagonists are criminals and psychopaths. For example, in Cornell Woolrich's "A Bride Wore Black," the heroine is a serial-killer taking revenge on the five men she blames for the death of her husband.
Action needs to be at the core of a fulfilled piece of noir fiction, and the more unpleasant and rotten the action the better your noir fiction will be. Cormac McCarthy creates one of the most wicked killers -- he executes his victims with a tool used to kill cattle -- in his novel "No Country For Old Men." When a character is capable of atrocious action, there is an incredible drive to the prose.
There are few wants more sinister than brutal revenge, and it is a feeling that can be used in order to develop your noir fiction plot. Whether one of your central characters wants revenge because of sex, money or murder, the degree of revenge is based on the degree of the perceived or actual wrong. The character's desires motivate his actions.
No Happy Endings
When you create sinister characters, your reader wants them to meet equally evil ends. This doesn’t always have to be death. The noir fiction genre is based on the awful realities of the world, and whether this means your central character will be slowly crushed by a boulder or forced to listen to a squealing pig for hours on end, your ending must surprise your reader.