Warhol's film work falls into a silent and a sound phase, the first of which reached its apex in "Sleep" (1964), a six-hour study of a slumbering man conveyed via a virtually stationary camera. Glacially indifferent to the question of viewer involvement, "Sleep" is not so much "watched" as it is "experienced."
Warhol was prolific in his idiosyncratic, voyeuristic brand of "cinema verite", churning out product at an assembly-line clip of roughly one film a week during the period 1964-65. He trained his camera on the motley band of freaks, musicians and social register slummers that trooped through his Felliniesque "Factory". In an ironic inversion of the Hollywood studio system, Warhol elevated the more prominent "players" into underground "superstars": the beautiful, tragic Candy Darling, Joe Dallesandro, Holly Woodlawn, Jackie Curtis, et al.
Although all of Warhol's films were governed by his peculiar sensibility, he assembled a nucleus of capable technicians, such as Paul Morrissey and Chuck Wein, who made various--uncredited--contributions, often in the master's absence.
Warhol entered his "sound phase" with "Harlot" (1965) and continued to crank out such influential films as "Vinyl" (1965), based upon Anthony Burgess's "A Clockwork Orange", which launched the tragic career and cruel exploitation of socialite/superstar Edie Sedgwick. In 1966 he produced his most enduring and definitive work, "The Chelsea Girls", a crazed showcase of Factory stalwarts which synthesized the enthusiasms and strategies encompassed by his previous work. The film was projected on two adjacent screens, each of which depicted unrelated situations. Its relative popularity ("The Chelsea Girls" was the first Warhol film to surface in "real" movie houses), inspired a more commercial, or at least less arcane, approach to filmmaking.
While such post-Chelsea Girls" films as "Lonesome Cowboys" (1969) continued to use typically Warholian alienation effects (extreme long takes, "strobe" cuts, etc.), they also relied on previously disdained qualities such as plot and characterization. By the time the Factory closed, after an attempt on Warhol's life in June of 1968, Morrissey had inserted his more formal concerns into the Warhol formula, producing a series of bizarre sex farces that proved more accessible to a popular audience (although they gradually reverted into self-parody). By the mid-1970s, Morrissey was turning out Gothic romps affixed with Warhol's brand name, although they were only vaguely indebted to the Factory style.
Though he had effectively closed the filmmaking chapter of his career after the release of "Andy Warhol's Bad" (dir. Jed Johnson, 1977), Warhol continued to satisfy his voyeuristic appetites with a Polaroid camera that he toted on his late-night revels until his untimely death in 1987.