As J.W. Rinzler observes in The Making of The Empire Strikes Back, filmmakers in 1979 contemplating the prospects of a blockbuster sequel didn’t have any reason to feel confident of success.

In 1970, Beneath the Planet of the Apes had taken in about half of the first film’s profits; Godfather II (1974) had made far less than half of its predecessor’s U.S. gross. More recently, in 1978, Jaws 2 had successfully cleared $100 million — but that figure was not even close to 50 percent of what Jaws earned — while James Bond in 1977’s The Spy Who Loved Me faltered with less than $50 million domestically.

As daring as Star Wars was, The Empire Strikes Back was an even more audacious gamble. The sequel landed in theaters 40 years ago today, with posters declaring THE STAR WARS SAGA CONTINUES. The new film wasn’t just a second Star Wars, it was the next installment in an ongoing series.

Star Wars had been retrospectively numbered Episode IV, and Empire would be Episode V. Then there had to be at least an Episode VI, because Empire would end on a cliffhanger. Ultimately, Lucas was already imagining, Episode I, II, and III would chronicle the conflict between Darth Vader and Obi Wan Kenobi; Episode VII, VIII, and IX would pick up the Skywalker saga in later years.

Remarkably, that’s essentially what happened, if not quite on the terms Lucas originally imagined. (One of the first things Disney did after purchasing Lucasfilm in 2012, to Lucas’s chagrin, was to shelve the founder’s outlines for post-Jedi films.) When The Empire Strikes Back was in production, though, that outcome was far from certain.

Lucas had literally bet the ranch on the sequel. An independent spirit who was tired of studio meddling, he self-financed the production of the second Star Wars, spending all his profits from the first film and more. Empire wouldn’t just have to be a hit, it would have to be another juggernaut on the level of Star Wars to allow Lucas to achieve his dream of establishing a self-contained filmmaking community on Skywalker Ranch. There have been plenty of successful sequels to hit movies since, but none have meant so much.

Gearing up to make a second Star Wars, Lucas made a decision that would be fundamental to the nature of the final film: he delegated directing duties to Irwin Kershner, an older moviemaker who Lucas had admired since film school. While Empire would be Kershner’s only foray into the Star Wars universe, his impact on the franchise as we know it today can’t be underestimated. Patient, painstaking, and firmly focused on the human story, Kershner ensured that Episode V would rise to the mythic heights Lucas envisioned. The love story works, the family drama works, and — crucially — Yoda works.

Everyone involved knew that Empire would live or die on the diminutive shoulders of the puppet sculpted by Stuart Freeborn and acted by Frank Oz. Lucas was understandably nervous about letting the voice of Fozzie Bear speak for a Jedi master, and Kershner was not amused when Mark Hamill talked the puppeteer into sneaking Miss Piggy onto Dagobah for a surprise pop-up appearance during filming in Yoda’s hut one day.

As with much else about Empire, Yoda was conceived with extraordinary ambition: if he flopped, he’d flop, but if he worked, he’d really work. It took multiple people working together to move the Jedi master and animate his face, and with Oz underneath the set, Hamill couldn’t even hear the puppeteer delivering Yoda’s lines. (An experiment with a radio earpiece to feed the dialogue to Hamill was abandoned when commercial radio signals kept cutting in. “If you turned your head the wrong way,” the actor explained, “you’d pick up Radio 1 and the Rolling Stones singing ‘Fool to Cry.'”

To make room for the vast jungle planet set, as well as the Rebel hanger on Hoth, an entirely new “Star Wars Stage” was constructed at Elstree Studios in England. Production was delayed, though, when fire broke out on the set of the film that preceded Empire at Elstree: Stanley Kubrick’s The Shining. An entire stage burned down, and since Kubrick had to regroup on another stage, “virtually two stages were denied to the Star Wars team,” reported unit publicist Alan Arnold in his book Once Upon a Galaxy: A Journal of the Making of The Empire Strikes Back.

Arnold’s dutiful labors resulted in priceless documentation of the process of making the movie. He interviewed just about everyone on set, several of the principals multiple times; remarkably, he even managed to mic Kershner on the day the now-iconic carbon freezing scene was set. The recording reveals tension involving Carrie Fisher, who was understandably upset that Kershner and Harrison Ford had revised the scene (including the legendary “I know” line, Ford’s idea) without consulting her.

Billy Dee Williams, a highly professional veteran of stage and screen, was growing impatient with the less disciplined Fisher, who tested a potential Lando slap by whacking Williams in the face without bothering to fake it. Meanwhile, while Kershner was trying to get the fog right and keep all the Stormtroopers in place without falling off the elevated platform, David Prowse was trying to tell the director all about his new fitness book.

Prowse, who acted the part of Darth Vader in all three of the original Star Wars films, was famously loose-lipped when it came to leaking plot secrets — so it was absolutely imperative that he not be allowed to know the script’s most closely guarded twist, the revelation that Vader was Luke Skywalker’s father. He was fed a dummy line, knowing he’d be overdubbed by James Earl Jones but not knowing exactly how. Hamill, one of only a few people on set who knew what the true line would be, had to act the scene without his costar knowing what he was reacting to.

The film was physically and mentally exhausting for Hamill, who had far more time onscreen than any other actor and did most of his own stunts. Production had to be temporarily shut down after he sprained his wrist filming the scene where Luke leaps away from the snowspeeder being crushed by an AT-AT. Hamill was also bit by a snake on Dagobah, fell off the catwalk where was battling Prowse, and had heavy debris heaved at him during the scene where Vader uses the Force to attack Luke with projectiles.

On top of that, Hamill and much of the rest of the crew battled frostbite when filming the Hoth exteriors on a glacier in Finse, Norway — a location shoot in a climate that was, if anything, even more hostile than it looks on film. A snowstorm slowed production for so many days that Ford, who was not originally going to be part of the Norway shoot, was summoned to shoot his scenes as Han Solo in the snow. The landscape was so oppressive, many scenes could be shot right outside the crew’s hotel.

Having revolutionized visual effects on Star Wars, Lucas knew the bar was even higher for Empire. While principal photography took place in England and Norway, back in California the Industrial Light and Magic team labored to bring the alien environments alive with techniques ranging from matte paintings to stop-motion animation, which was used for the Imperial walkers and the tauntauns.

In one remarkable example of the team’s ability to solve seemingly any problem, effects supervisor Dennis Muren worked out the math to shoot a stop-motion tauntaun in a sweeping camera move to precisely match a helicopter shot from Norway that Lucas identified as the best one to introduce viewers to the Rebels on Hoth. Sound designer Ben Burtt dispatched a colleague to metalworking plants around the Bay Area to record the sounds that would be altered and combined to make the AT-ATs sound like mountains of metal advancing on Echo Base.

The result was a film that recaptured the escapist fantasy of Star Wars, but with a stronger human story — thanks in large part to screenwriter Lawrence Kasdan, who came in to help Lucas rewrite the first draft by Leigh Brackett, the noted science fiction author who died shortly after she handed it in. Empire also recaptured the knack of the burgeoning franchise for evoking unforgettable environments. Cinematographer Peter Suschitzky found stunning beauty on the imposing ice planet, in Yoda’s mystical swamp, on the Deco-inspired Cloud City, and in outer space, where Han and Leia shared their first kiss in a seemingly quiet corner of the Millenium Falcon. (Having Threepio interrupt the embrace was an inspiration by Kershner.)

The film’s stunning success helped Lucas expand his enterprise — including the computer division that would become Pixar. While ILM used computers on Empire for shot tracking and camera programming, the film became a touchstone of the lovingly handmade aesthetic Disney-era Star Wars has strived to recapture, for example in using puppetry rather than computer animation to create the diminutive droidsmith Babu Frik in The Rise of Skywalker.

In the eyes of many, Star Wars would never again reach the heights of Empire. Although Lucas talked at the time about getting back to making “personal” films, and was loath to steal Kershner’s thunder as the film’s director, history will remember him as the author of the Star Wars saga — and it was The Empire Strikes Back that made “saga” a word the franchise had well and truly earned.

– Jay Gabler