The primary moods of classic film noir were melancholy, alienation, bleakness, disillusionment, disenchantment, pessimism, ambiguity, moral corruption, evil, guilt, desperation and paranoia. Heroes (or anti-heroes), corrupt characters and villains included down-and-out, conflicted hard-boiled detectives or private eyes, cops, gangsters, government agents, socio-paths, crooks, war veterans, petty criminals, and murderers. These protagonists were often morally-ambiguous low-lifes from the dark and gloomy underworld of violent crime and corruption. Distinctively, they were cynical, tarnished, obsessive (sexual or otherwise), brooding, menacing, sinister, sardonic, disillusioned, frightened and insecure loners (usually men), struggling to survive - and in the end, ultimately losing.
The females in film noir were either of two types - 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. Often, it would be to follow the goadings of a traitorous femme fatale who destructively would lead the struggling hero into committing murder or some other crime of passion. 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 and death.
Film noir films (mostly shot in gloomy grays, blacks and whites) showed the dark and inhumane side of human nature with cynicism and doomed love, and they emphasized the brutal, unhealthy, seamy, shadowy, dark and sadistic sides of the human experience. An oppressive atmosphere of menace, pessimism, anxiety, suspicion that anything can go wrong, dingy realism, futility, fatalism, defeat and entrapment were stylized characteristics of film noir. The protagonists in film noir were normally driven by their past or by human weakness to repeat former mistakes.
Film noir was marked by expressionistic lighting, deep-focus camera work, disorienting visual schemes, jarring editing or juxtaposition of elements, skewed camera angles (usually vertical or diagonal rather than horizontal), circling cigarette smoke, existential sensibilities, and unbalanced compositions. Settings were often interiors with low-key lighting, Venetian-blinded windows and rooms, and dark, claustrophobic, gloomy appearances. Exteriors were often urban night scenes with deep shadows, wet asphalt, dark alleyways, rain-slicked or mean streets, flashing neon lights, and low key lighting. Story locations were often in murky and dark streets, dimly-lit apartments and hotel rooms of big cities, or abandoned warehouses. [Often-times, war-time scarcities were the reason for the reduced budgets and shadowy, stark sets of B-pictures and film noirs.]
Narratives were frequently complex, maze-like and convoluted, and typically told with foreboding background music, flashbacks (or a series of flashbacks), witty, razor-sharp and acerbic dialogue, and/or reflective and confessional, first-person voice-over narration. Amnesia suffered by the protagonist was a common plot device, as was the downfall of an innocent Everyman who fell victim to temptation or was framed. Revelations regarding the hero were made to explain/justify the hero's own cynical perspective on life. Some of the most prominent directors of film noir included Orson Welles, John Huston, Billy Wilder, Edgar Ulmer, Robert Siodmak, Fritz Lang, Otto Preminger, and Howard Hawks. Titles of many film noirs often reflect the nature or tone of the style and content itself: Dark Passage (1947), The Naked City (1948), Fear in the Night (1947), Out of the Past (1947), Kiss Me Deadly (1955), etc.