The Exorcist uncut: Secrets of the scariest movie ever made

The Exorcist has been terrifying audiences for decades. But the dark secrets of its making are only now coming to light. Guy Adams reveals the truth of a horror classic

A teenage girl, horrifically disfigured, sits up in bed and projectile-vomits dollops of thick, green gloop onto the face of a priest. Then, in a stream of appalling profanity, the girl confirms what we already guessed: that she happens to be possessed by the Devil. As plot twists go, it's not exactly clever or sophisticated. But the scene is nonetheless hugely effective: this is, after all, one of the climactic moments of The Exorcist, which in turn is among the most terrifying and brilliantly-executed horror movies.

Each year, as Hallowe'en approaches and Hollywood tries to bridge the gap between the summer and Christmas blockbuster seasons by making us scream, I'm reminded of William Friedkin's 1973 masterpiece. With its rotating heads and shaking beds, it's easy to dismiss it as crude and unsubtle; at times, it even teeters on the brink of self-parody. But, when you watch it closely, you realise it's a hugely-accomplished piece of movie-making. That's why it still puts the fear of god into people.

This week, a Blu-ray version comes out, containing not just all the usual digital remastering that accompanies expensive "collectors' editions", but also an utterly fascinating new documentary based on behind-the-scenes footage taken from the sets in Georgetown and New York. Shot on a wobbly hand-held camera, it has for 37 years laid undiscovered in the garage of Owen Roizman, the director of photography later short-listed for the Best Cinematography Oscar (one of ten nominations the movie garnered) for his work on its ten-month shoot. Roizman's short film lays bare perhaps the two most important factors behind The Exorcist's success: firstly, the extraordinary lengths to which Friedkin's technical staff went – in an era long before the advent of CGI – to achieve the extravagant special effects on which its narrative relies; and, secondly, the canny decision to give the production a pared-down, fly-on-the wall quality which would influence dozens of more recent horror hits including The Blair Witch Project, and Paranormal Activity.

Without either of those factors in play, the film, adapted from William Peter Blatty's novel, could very well have been an overblown disaster. Though originally based on a real-life tale, there are times when it seems so dark, and its dialogue so extravagant, that a more stylised production would have been little short of comic. "This film was just begging to be a flop," is how Friedkin recently put it. "If we'd got it wrong, it could very easily have turned into a laugh-riot."

Instead, it was utterly convincing. On a commercial level, the tale of Regan, a teenage girl who becomes possessed by Satan after playing with a Ouija board, has made almost half a billion dollars for Warner Brothers, which originally backed it with a (then enormous) budget of $10m. On an artistic one, it has come to define the career of Friedkin, an obsessive and often difficult individual who approached the making of The Exorcist in the manner of a mildly unhinged dictator going to war..

The documentary tells how sometimes, just before cameras rolled, Friedkin would fire guns, so that his stars looked startled. At other times, he'd slap the actors across the face to make them appear angry. That directorial tactic prompted actress Ellen Burstyn, who played Regan's mother, to describe him "a maniac". Before every take of the film, he would meanwhile insist on playing unsettling music on loudspeakers.

Friedkin also required his entire set to be refrigerated, so that viewers would be able to see the breath of characters freeze during exorcism scenes. "Today, doing that would be a piece of cake, right?" he says. "Today they can make you believe that the Titanic is sinking." Back then, it was a hugely expensive and uncomfortable operation. In Roizman's behind-the-scenes footage, you therefore see crew-members creeping about the set in 1970s ski jackets, while Burstyn and Linda Blair, who won a Golden Globe for her portrayal of Regan, snivel miserably in the cold.

You also learn how charmingly low-tech most of The Exorcist's other special effects were, and how remarkable it was that they still manage to look even slightly convincing on screen. The famous scene when Regan's head rotates through 360 degrees, for example, was filmed using a life-size rubber model of the actress. The model looks hopelessly artificial in broad daylight, when Roizman's backstage footage shows Blair playing around with it, but utterly convincing when placed on a carefully-lit set.

For the scene where she vomits, an artificial device was strapped onto Blair's chin, and which used a hidden tube to fire a jet of green liquid, made by mixing of pea soup and porridge. Today, most of Friedkin's other techniques also seem extraordinarily outmoded. To shoot scenes at the angles he desired (in an era that predated "steadycam" devices), he required staff to erect a bewildering array of pulleys and wires, which the cameramen would simply be dangled from. To make the bed, on which Regan sits for much of the second half of the film, rock violently, his crew installed a Heath Robinson-style mechanism powered by four men who stood backstage pumping levers.

"Seeing what went on all those years ago has reminded me that this truly was, as we have been saying for so many years, the greatest magic act ever filmed," said Blair, who was just 13 when it was filmed, at the launch of the documentary in New York. "What Billy Friedkin did on that set was magic. That's what I always tell people; there's simply no other way to describe it."

A great deal of The Exorcist's success also derives from Blair's remarkable performance. One is struck by the patience with which she endured hours of often uncomfortable make-up. It's also extraordinary watching how Blair would lark around until almost the moment the camera rolled, just how brilliantly she was able to turn her character on and off, and how superbly she was choreographed. Today, Blair gives Friedkin huge credit for effectively gambling his professional reputation on a film that depended on the performance of small child. It required endless patience. "Sometimes I wouldn't want to do something. And he would have to make it a game. Or he would offer me some sort of benefit. 'Would you like to have a chocolate shake?' he would ask. 'If you finish this then you can have one.' And it worked."

A similarly patient approach gave The Exorcist its uniquely unsettling sound effects. As the film progresses, for example, Regan's voice morphs into an elaborate sort of cacophony. It turns out that some of the sounds used to realise this were supplied by Blair, others by a voiceover artist called Mercedes McCambridge. Still more came from hundreds of other recordings of an array of diverse sounds, including croaking tree frogs, and bumblebees.

"Billy [Friedkin] and I had many discussions before filming," says Chris Newman, who was in charge of sound. "The big problem was that there's no common language to describe how Satan should sound. You could say, for example, 'who's an evil person?' and someone might answer 'Hitler!' but that doesn't mean you can give the Devil a German accent. Eventually, Billy came into a meeting with a book, and showed me that Hieronymus Bosch painting called The Garden of Earthly Delights. And he pointed at all the little characters in the picture, and there are dozens of them, and said: 'that's what the voice of Satan should sound like'. In many ways, with what we ended up doing, I think it does."

In recent times, a popular mythology has built up around The Exorcist. Its production was famously said to be cursed: a mysterious fire devoured one soundstage and no less than nine people connected to the project died during filming. At one point, Father Tom Bermingham, a Catholic priest who worked as technical advisor to Friedkin, agreed (at the crew's request) to perform a ceremony to rid the set of malign influences.

There was certainly plenty of suffering involved. Burstyn and Blair were left with longstanding back problems because of the extent to which they were thrown around on the mechanical bed. A stuntman was left black and blue after throwing himself down 90 steps to film the final sequence. And endless members of the crew were fired, or resigned, after refusing to play ball with Friedkin.

"Billy and I had a relationship, so I wasn't afraid of him," recalls Roizman. "But a lot of people were. He did fire plenty of them, and a lot of other people quit. The make-up artist Dick Smith quit three times, for example. Each time Billy had to talk him into staying. He can be the most charming guy in the world when he wants to, but he's also the biggest schizophrenic I know. Completely warm one minute, and just venomous the next."

Priests have, for hundreds of years, talked about Satan in similar terms. It was Friedkin's ability to assume the mercurial characteristics of the Devil he was attempting to portray that made The Exorcist such an extraordinarily-unsettling piece of cinema.

'The Exorcist: Extended Director's Cut and Original Theatrical Version' is out now on Blu-ray