A tone of uncertainty and disillusioned cynicism tempered the relief and optimism that came with the end of the war. The United States became very powerful and Hollywood reflected these changes. A new sort of movie began to evolve that caught on with a public in search of dark thrills and hard bitten heroes.
"The Big Sleep" has all the elements that define film noir, it contains the irreverent protagonist, the femme fatale, tough guys and an atmosphere saturated with shadows.
Film noir is almost more of a mood and an atmosphere than a style or genre, it roots in both German Expressionism and the American hard boiled detective fiction of the 20's and 30's. Informed by the traditions of 30's gangster and social consciousness films, writers such as Chandler began to expand on the traditional crime genre and blur the black and white morality of those conventions.
"The Big Sleep" deals with pornography, drugs, a censored opium den, homosexuality and nymphomania, which even though heavily censored due to the Hays code reflects elements of crime fiction and film noir. .
Film noir redefines language with its camera angles, lighting and editing. For example, it experimented with scenes and styles with rooms divided by jagged angular patterns of light, oblique shadows, grimy city streets and alleys slick with rain.
Chandler's cynical, hard boiled private eye, Philip Marlowe, set the standard for tough brooding heroes with a strong sense of moral conviction living in a cruel and indifferent world, Marlowe: I could make it my business, Mars: I could make your business mine, Marlowe: You wouldn't like it the pay's too small.
The film is directed so that we see everything Marlowe sees. This shows the audience that he is independent, prepared and can handle situations under pressure. We are also shown that the hard boiled detective treads a fine line of ethics e.g. Marlowe plays by his own rules when he steals evidence from the police.