Primary Characteristics and Conventions of Film Noir:
The primary moods of classic film noir were melancholy, alienation, bleakness, disillusionment, disenchantment, pessimism, ambiguity, moral corruption, evil, guilt, desperation and paranoia
The females in film noir were either of two types (or archetypes) - dutiful, reliable, trustworthy and loving women; or femme fatales - mysterious, duplicitous, double-crossing, gorgeous, unloving, predatory, tough-sweet, unreliable, irresponsible, manipulative and desperate women. Usually, the male protagonist in film noir wished to elude his mysterious past, and had to choose what path to take (or have the fateful choice made for him).
Invariably, the choice would be an overly ambitious one, to follow the dangerous but desirable wishes of these dames. It would be to follow the goadings of a traitorous, self-destructive femme fatale who would lead the struggling, disillusioned, and doomed hero into committing murder or some other crime of passion coupled with twisted love. When the major character was a detective or private eye, he would become embroiled and trapped in an increasingly-complex, convoluted case that would lead to fatalistic, suffocating evidences of corruption, irresistible love and death. The femme fatale, who had also transgressed societal norms with her independent and smart, menacing actions, would bring both of them to a downfall.