“The horse is dead.”

It’s the middle of a tense scene in the 1969 smash Butch Cassidy and the Sundance Kid. The town sheriff is attempting to round up a posse to track down Butch and Sundance, leaders of the Hole-In-The-Wall Gang, which has been robbing banks and trains with such impunity that they’ve become an embarrassment for lawman across the frontier. Unbeknown to everyone, these celebrity outlaws are watching the scene unfold from a perch across the street, where they’re blowing their loot on liquor and whores, but the sheriff’s recruitment efforts were doomed to run aground regardless. There just isn’t much appetite for going after an elusive and dangerous pair that seem to be generous in spreading their stolen loot around.

There is, however, time for a brazen salesman to make a pitch: “The horse is dead,” he says. The future is bicycles.

The sequence everyone remembers from Butch Cassidy and the Sundance Kid follows shortly afterwards, when Paul Newman, as Butch, pedals Katherine Ross’s Etta Place around on the handlebars of a new bike, with the jaunty Oscar-winning song Raindrops Keep Fallin’ on My Head playing in the background. It may not be accurate to call the film a “revisionist western” – a term reserved for less commercial visions like Monte Hellman’s The Shooting, Robert Altman’s McCabe & Mrs Miller, and Sam Peckinpah’s The Wild Bunch, released the same year – but even before our heroes meet a hail of bullets in the final freeze-frame, the end of an era is nigh. It’s no wonder Butch winds up chucking the bike into the woods later on.

Butch Cassidy was a box-office sensation when it premiered 50 years ago, more than doubling Midnight Cowboy and Easy Rider, which grossed $44.8m and $41.7m, respectively, against its colossal $102.3m. For perspective, only six films since have doubled up the competition since – three of them Star Wars movies (Star Wars, The Empire Strikes Back, and Return of the Jedi); two of them Steven Spielberg movies (Jaws and ET); and the other Titanic. It’s important to understand the film as a commercial juggernaut, first and foremost, because it’s so plainly eager to turn a horse into a bicycle, reconfiguring the hidebound traditions of the western for a hipper, more self-aware audience.

In a William Goldman script peppered with memorable lines, the first exchange sets the tone. Butch looks around a bank at closing time, chatting with the security guard as he perhaps sizes up his next job.

“What happened to the old bank? It was beautiful.”

“People kept robbing it.”

“That’s a small price to pay for beauty.”

Photograph: Sportsphoto/Allstar

Right away, Goldman establishes Butch as a charismatic mouthpiece for the quip-ready screenwriter, contrasting nicely with the Sundance Kid, Robert Redford’s taciturn sharpshooter. But he’s also created two heroes who break the western mold, neither justice-seeking white-hats nor grizzled, sneering black-hats, and not as traditionally masculine as either party. Butch is a man who appreciates beauty and art, but doesn’t have the stomach for violence; it’s not until late in the film that we (and the Kid) discover that he’s never shot a man before and he looks sickened to have to do it. He’s a pleasure-seeker above all else: robbing banks and trains are his way to make an easy living and enjoy whatever sinful freedoms his vocation affords him.

Audiences in 1969 were all too happy to embrace the light, quippy irreverence of Butch Cassidy and the Sundance Kid after a turbulent summer, and Goldman, director George Roy Hill, and the two impossibly handsome stars made them feel cool for doing it. True Grit had performed well earlier in the year as a throwback to the genre’s past, giving John Wayne a proper victory lap, but Butch Cassidy was thoroughly modern, a star-making vehicle for Newman and Redford that reflected a need for the genre to turn the page and that feels as much of its time as it does authentic to Wyoming in the late 1890s. With Ross at the center of a love triangle between friends, the film attempted to bring Jules and Jim to the American mainstream, taking a lesson from the French new wave on how to revivify old Hollywood craft.

It still works spectacularly well. There’s an alchemy up and down the production: Jack Lemmon, Steve McQueen, and Warren Beatty all passed on playing the Sundance Kid, and none seem capable of the quiet confidence that Redford possesses in the role, which parries so well with Newman that the two would run it back again with Hill a few years later in The Sting. The pop doodling of Burt Bacharach’s score is about as far from a traditional western score as possible, but it somehow meshes with the sepia sheen of Conrad Hall’s photography, which burnishes the legend of these two men while their story is still being told. And while Goldman’s screenplay dances on the edge of glib, it’s lively and sophisticated, with a strong theme about the capitalist forces that really tamed the Wild West.

Butch Cassidy and the Sundance Kid is such a rollicking good time that it takes a while to notice it’s about the end of the line for its heroes, whose celebrity is already widespread when the film opens and ultimately hastens their demise. “Your times is over and you’re gonna die bloody,” warns a sheriff, prophetically, in an early scene, and the film is mostly about Butch and Sundance getting chased out of America by hired guns and dying at the hands of the Bolivian army. They’re mostly guilty of stealing from the wrong guy: EH Harriman, the railroad tycoon, spends more trying to catch them than they rob from his safes, but it’s an opportunity for a powerful man to send a message about who’s really in charge. Guys like Butch and Sundance can handle local lawmen and half-hearted posses, but they can’t fight progress. The EH Harrimans of the world will make certain of that.