"So now, because of a bogus reputation, they will not read The Exorcist," he says. "They will not look at the film. They're terrified."
It is a sad irony that a man with Blatty's background should find himself being painted as the Devil's champion. He was raised as a Catholic by a mother who, he says with admiration "was extremely devoted to the Blessed Mother"; and his great-uncle was an archbishop in the Middle East. Religion is in the genes of William (or Bill, as he rather sweetly prefers to be called), so there came a time when he himself considered entering the priesthood.
"It was a strong possibility," he says with a sigh. "But I was too fond of the ladies; a vow of chastity was not going to work for me."
Instead, Blatty became a writer with a gift for comedy. Using laughter to exorcise the painful circumstances of his poverty-stricken childhood, he transformed some of his own "horribly painful experiences" into his first comic novel, Which Way to Mecca, Jack? In 1963 he broke into screenwriting, with the Danny Kaye vehicle The Man From the Diners Club. A script for Inspector Clouseau's second outing, A Shot in the Dark, followed. After several more films, Hollywood turned its back on comedy and Bill found himself out in the cold.
"I was put up for straight dramatic writing jobs time and again, but I couldn't get a job. So I had nothing better to do than go down and collect my unemployment cheque once a week and start working on this novel [The Exorcist]. I had been thinking about the idea for 15 years, but I had never dared to attempt something serious..."
Of course, the novel and its big-screen spin-off became a phenomenon, the latter earning Blatty an Oscar for his screenplay. He says with sadness, though, that the only impact of the award was that: "I stopped worrying about how I was going to pay my bills. I never felt a rapture or even a glow of success. The process took too long. From the time I thought of the idea in college, then 20-25 years later taking months, if not years, to get someone interested in it, and then the writing of it... By the time it all came to pass, my primary emotion was just one of immense relief."
The Exorcist reflects Blatty's own doubts about religion. "When I was writing those scripts years I was having my own crisis of belief," he says slowly. "As the characters worked through their problems of faith, I was working through mine."
But the film went further than he'd anticipated. Although Blatty had done his best in his script to ensure that people understood the outcome, the excision of two cherished scenes by William Friedkin made it possible to misconstrue the priests' victory as defeat. The scenes gave the film a moral centre, says Blatty; and they gave you "an opportunity to not dislike yourself for liking the movie". But Friedkin decided that the film was running too long, and "the theology was the first candidate to go out the window".
Seventeen years later, Blatty got an opportunity to clear up the misunderstanding when he directed his first feature, The Ninth Configuration. Based on an early comic novel, Twinkle, Twinkle "Killer" Kane, the film ends with an uplifting coda that is an affirmation of Kane/ Blatty's religious faith. If Blatty's intended message - "God is in his Heaven: all's right with the world" - got lost in Friedkin's film, there's no mistaking it here.
"Doing The Ninth Configuration, I welcomed the chance to clarify where I stood," he says. "And to show all those people who had misunderstood The Exorcist that I was not the Antichrist disguised as an author."
The Ninth Configuration also gave Blatty an opportunity to reconcile, for the first and last time in film, the different strands of his career. Prior to that novel, comedy had been his forte and his first love; after it: "Nobody wanted comedy from me any more. It's as if I had landed on the planet with the manuscript of The Exorcist under my arm... That's true even today. If I do mention that I wrote A Shot in the Dark, their eyes glaze over and 10 minutes later they've completely forgotten it."
Nevertheless, Blatty still hopes that he will be asked to direct a comedy, and will not just be offered films like "Pumpkinhead 4. Or was it Pumpkinhead 3? That's the kind of offer I get."
Behind this wry, self-deprecating humour, the warm and engaging Blatty is a little disappointed with the way he is perceived today. During our conversation, he seemed almost pitifully grateful to be given the opportunity to talk about a career that has far more shading than most of us realise. Once it was religious reassurance that he needed. Today, it is the reassurance that he will actually be remembered as more than just the author of The Exorcist which Blatty appears to require most.
`Ninth Configuration' is released this week