Stories surrounding The Exorcist are legion. Nine production staff died during its shoot, an unexplained fire tore through the set and punters fainted in the cinemas when the film was finally sprung on the outside world. Writer William Peter Blatty got off more lightly, but even he suffered a loss.

Prior to The Exorcist, he was best known as a comedy writer, but the film's huge success killed that career as dead as a doornail. "I was, for one brief, shining moment, the industry's top comedy screenwriter," shrugs Blatty, whose work included the Danny Kaye comedy The Man From Diner's Club and the Pink Panther sequel A Shot in the Dark, starring Peter Sellers. "But then I got this silly idea of writing The Exorcist."

In its various incarnations (bestselling novel, top-grossing film), The Exorcist has possessed Blatty for some 30 years now. Pigeonholed as a writer of serious, theological horror stories, he has never been able to return to his first love. "Never. People would blanche when I said I wanted to. I finally got an obscure imprint to publish Demons 5, Exorcist 0, which was basically an Exorcist spoof, and it went absolutely nowhere."

In person, Blatty seems to embody the two sides of his craft. Physically, he is a little spooky, with his 71-year-old face, jet black hair and bright blue eyes. Personally, he is a witty, irrepressible raconteur, braying deep-throated laughter into his cup of tea. He says he felt imprisoned by The Exorcist for years, but is now resigned to it becoming his legacy (though he feels his later film, The Ninth Configuration, represents his best work).

Dubbed "Satanic" at the time by Billy Graham, The Exorcist set new standards in movie horror with its powerfully researched, brilliantly played tale of demonic possession. Twenty-six years after its initial release, the picture still resonates like a depth charge. "I've always thought that there is a mystery to the film, in that its effect on people is greater than the sum of its parts," Blatty points out. "Billy Graham was on to something when he said that there was evil in that film. He meant evil down deep in the very celluloid and I agree with him in one extent. I'm not sure it's evil, but it has an otherwordly power that hits you in the viscera."

The current Director's Cut of the film restores some of Blatty's original material. There is an additional scene of Regan (the possessed girl memorably played by Linda Blair) in the hospital and a subtly modified, more spiritual ending. Director William Friedkin says that while Blatty has been arguing for these changes since the film's original release, it took him 26 years to come round to the writer's way of thinking.

Blatty feels vindicated. "The spiritual core is restored, which is important not just for its own sake, but in terms of audience satisfaction because you can enjoy the film without despising yourself. Plus it's richer, it's more complete. People have also said that it's creepier too." He shrugs. "I don't know. I have no opinion about that."

Blatty, it turns out, is very resistant to the perception of his film as a horror movie. He sees it as a deeply religious film, one that reflects his own strict Jesuit upbringing; Blatty's great uncle was an archbishop, his Lebanese-born parents were devoutly religious and Blatty himself still attends church every Sunday.

But having said all that, there is little that is po-faced or pious about the man. One of the most bizarre legends about Blatty concerns his TV appearance on the Groucho Marx game show You Bet Your Life. Blatty wound up winning $10,000 ("I also won a carton of Old Gold cigarettes") and said he was going to use the money to write a novel. The rest, as they say, is history.

But the story takes a new twist when the writer tells it. Blatty went on the game show disguised as a Saudi prince, the son of King Saud. "Groucho knew my real identity," he laughs. "No-one else did." In actual fact, Blatty posed - on and off - as the prince for nearly a year, gathering material for a humorous article ('I Was an Arab Prince') that eventually appeared in the Saturday Evening Post. In this disguise, Blatty (who was then working as a publicist) became the toast of Hollywood, feted by the great and the good and made the guest of honour at various parties and nightclubs.

"The prince would always get the top table," he remembers. "The management would come over and ask what his favourite song was so that the band could play it. The prince would say that his favourite song was Danny Boy." Hearing him tell the tale is both funny and oddly poignant. You can catch a glimpse of the way he used to be, the mischievous jester soon to be overtaken by The Exorcist. At least young Regan got herself exorcised. Blatty, you suspect, has had to make peace with his demon.

 The Exorcist: The Director's Cut opens November 17.