A lot can be said about Sam Peckinpah's western 'Pat Garrett and Billy the Kid,' on this, the 40th anniversary of its release, but you have to start with the fact that the story behind this startling, savage movie is just about as startling and savage as the film itself.

Like the time a camera lens got damaged without anyone realizing it while they were shooting in Durango, Mexico. Looking at the ruined dailies, an enraged Peckinpah, known for his alcoholic furies, stormed out of his director's chair and pissed all over the screen – leaving on it an S-shaped stain like the brand on a steer.

"Sam was drunk, of course," recalls Kris Kristofferson, who plays Billy. "By the end of the day that bottle had taken over. I'll never forget Bob Dylan turnin' and lookin' at me like, 'What the hell have you gotten me into?'"

Dylan wrote the songs for the movie, including "Knockin' on Heaven's Door." He also has a small role as the mysterious printer's assistant named Alias, who joins up with Billy and his gang. Rita Coolidge, Kristofferson's girlfriend at the time, plays a Mexican woman whom Billy/Kristofferson makes sulfurous love to just before he's gunned down by Pat Garrett (played by James Coburn), his erstwhile friend and mentor.

Peckinpah would start drinking on the set first thing each morning, and by the afternoon he would be loaded and walking around firing a revolver into the air. At night he'd lie in bed shooting at his reflection in the mirror, a drunken outburst that made its way into the movie when, after killing Billy, Garrett, in a bout of self-hatred and disgust, shoots to pieces his own reflection in a mirror. At one point, "I had to take a pistol away from Sam," says Kristofferson. "He was worrying some people."

It was that kind of production – and that kind of film, too: a twisted, paranoid tale of Billy's onetime outlaw friend, Garrett, becoming a pawn of the West's new commercial interests, and then hunting down Billy and killing him. Twenty years before Clint Eastwood's 'Unforgiven' mixed white hats and black hats into a murky gray sombrero, Peckinpah was throwing Molotov cocktails at the Hollywood western's conventional view of the upright American male.

A product of the Vietnam and Watergate era, when life and art danced and shrieked together in the streets, Peckinpah's film transformed the classic story of Garrett's viciously turning on young Billy into a tale of generational conflict. The middle-aged Garrett is the figure of the rotting American establishment, while Billy, by contrast, is vital and uncompromising, though just as vicious in defending his integrity as Garrett is in selling out.

At one point, Billy and another outlaw who has been deputized by Garrett step outside for a gunfight. They agree to face each other at 10 paces, but after his first step, Billy whirls around, waits for the other man to turn – at eight – and fatally shoots him.

Billy saunters toward the dying man, towering over him with a sad, rueful smile. "That wasn't 10, hoss," he drawls. The dying cowboy, played by the famed western character actor Jack Elam, replies, "I never could count." Like the movie itself, the scene undermines the romantic gunplay of the classic western, emphasizing instead the reflexive amorality and essential illogic of human nature.

Kristofferson felt the film's subversive power from the moment he read the script. Inspired by Peckinpah's vision, he helped bring Dylan into the project. That created an entire new set of problems.

"The whole thing was kinda weird for Bob," Kristofferson says, "because Sam was a pretty volatile character." To make matters worse, Peckinpah had never heard of Dylan. "Sam wasn't really that well-informed about music," Kristofferson says, joking that "he would just as soon have had Roger Miller or something."

In the end, though, Dylan's involvement all but guaranteed the film's immortality. "Sam was lucky that I made him use him," says Kristofferson. "The music that Dylan came up with was a major part of what made that film great." Despite the derangement of making 'Pat Garrett,' he fondly remembers Dylan sitting down at the piano after they wrapped each day, and "Bob and Rita doing music for hours, just singing song after song."

And in the end, for all the differences between Peckinpah and Dylan, the moment the tormented director heard Dylan sing, Kristofferson recalls, "he was in love with it."

In fact, Dylan's presence, from his gorgeous songs to his wry, gnomic face, lifted Pat Garrett far above what could have become another conventional installment in the Billy the Kid franchise.

The tired myth sorely needed a fresh approach. By the time Peckinpah snatched up the script for Pat Garrett, composed by a young screenwriter named Rudolph Wurlitzer, the inflated adventures of William Bonney, a.k.a. Billy the Kid, had been the subject of nearly 50 movies and dozens of stories, poems, and biographies. Years earlier, Peckinpah himself wrote a screenplay for a Billy the Kid flick to be directed by Stanley Kubrick. But both he and Kubrick got fired, and Marlon Brando eventually directed and starred in what became the equally powerful masterpiece 'One-Eyed Jacks.' Scenes from Peckinpah's script were used in Brando's movie, without credit, and the frustration of that experience must have helped drive him to make Pat Garrett.

Fabulous and weird – Peckinpah turns the legendary cowboy character actor Slim Pickens' death from a gunshot to the gut into a moment of beatific tranquility – unsparing in its psychology, searing in its incineration of the Hollywood western's starry-eyed morality, Pat Garrett has through the years been either unfairly dismissed as a deficient by-product of Peckinpah's self-destructive anger and drinking, or buried under the faintly praising epitaph: Here lies a film that was panned at the time but is now considered a classic.

Both judgments miss the mark. Pat Garrett's bracing originality shines through its sometimes shaky plot, and its spectacular brio repels the embalming status of "classic." The film explodes with the hallmarks of Peckinpah's noir-western style: the easy coexistence of virtue and violence; the fluidity of character, as Billy slips from romantic hero to cold-blooded killer and back again; the caprice of circumstance; the fascination with how and why people die violently. Peckinpah's film is a final twist, both astringent and loving, on a gasping genre. It wasn't merely a nice coincidence that the archetypal American cowboy John Wayne was making a movie at the same time right nearby. It was too good to be true.

Yet if the movie is an elegy for the western, it is also a lyrical demonstration of why the genre will never die. As Billy cuts off his shackles after killing his jailer, he sings a little song about all the places he's been, including the one he's escaping from. In the street, Dylan's Alias hears the tune and his face brightens with the slow dawn of revelation.

Suddenly a movie about the legend of Billy the Kid becomes a legend itself, a kind of ballad-movie, as actor/singer Kristofferson croons to singer/actor Dylan, the two young men sharing a common aspiration of finding their own destiny.

That small moment captures the movie's essence. Pat Garrett is about the timeless American showdown, that moment when an American youth can either choose responsible manhood or smuggle his boyhood into maturity in the attempt to fulfill his dreams. When asked if it was hard to stop being Billy after the movie was done, Kristofferson's answer is, "Shoot, I may still be." The toy pistols of our childhoods may have become PDAs, but the same goes for many of us, too.