I was watching the early sequences with a fair amount of dread. Then came what is perhaps the film's most extreme scene, in which the possessed pre-teen, Regan (Linda Blair), masturbates violently with a crucifix, smears blood on the face of her mother (Ellen Burstyn), and then attacks her telekinetically by making the furniture rush about. As a tallboy began its supernatural advance towards the cowering woman, my neighbour turned to me and said, quite matter-of-factly: "I do love American Colonial furniture, don't you?"
She wasn't trying to be disruptive, and the effect would have been less if she had. But from that moment, the film's mood collapsed and I could see only a pretentious freak show of lurid manifestations and phoney spiritual agonising. Without the support of the ambient hysteria, the film had no power.
A quarter-century after the film's first release, Friedkin is giving interviews about his most famous creation (eclipsing his first success, The French Connection, and overshadowing everything since). Not many people have the stamina for a single press tour, let alone two for the same piece of work, but Friedkin puts up a spirited and robust performance that only occasionally drops off into exasperation.
I had expected more ambivalence - no one has a good word to say for failure, but a surprising number of people end up feeling aggrieved by success. Friedkin seems not to be daunted by the prospect that he will be associated for all time with The Exorcist, rather than anything else he has done or will do. In the course of the interview, he plays strictly by the rules of engagement, mentioning in passing that the film is his only venture of its type, but never trying to shift discussion to his other work. Before we start to talk, he tries unsuccessfully to open the double-glazed windows - remarking that, no doubt, the seal is there to prevent residents of the Dorchester from attempting suicide - but that's the only sign that he would welcome a change of air.
Perhaps in years to come he will adopt the strategy of the novelist, John Braine, who in later life would, with masochistic humility, actually introduce himself with a mention of his only famous book (shaking hands with the words, "John Braine - Room At The Top"), hanging the albatross around his neck before anyone else could do it.
The Exorcist turns up regularly on lists of all-time Best Horror Films, but Friedkin insists that neither he nor William Peter Blatty, who wrote the screenplay and the novel on which it was based, thought in genre terms. "The Exorcist was not made to scare people in theatres. I frankly avoided all of the cliches of the classic horror films. All we ever talked about was a realistic film about `inexplicable things'."
It seems to matter to Friedkin that the novel was based on allegedly real-life happenings in 1949, which Blatty read about as a student. In his fiction, however, he changed just about everything - the period, the setting, and the gender of the possessed person. He doesn't go so far as to say that if the story hadn't had a link to reality, he wouldn't have been interested, but he draws a distinction between his film and pure fiction such as Rosemary's Baby.
There was considerable Catholic input into both novel and film, although Friedkin was agnostic. Yet for him the story has no sectarian aspects. "What happens in The Exorcist is no more astonishing to me than what happened in Nazi Germany, where it seems to me that an entire race of people was demonically possessed. I've read every imaginable book about what happened. There is no explanation I can offer that makes sense to me. Demonic possession on a grand scale, where ordinary people, went along with this slaughter of the innocents." He does admit that he himself has a capacity for evil, "up to a point", but resists the suggestion that this aspect might swallow him up.
A second viewing of The Exorcist makes it no clearer to me why the girl falls prey to possession. She dabbles in Ouija, but otherwise she's well- adjusted to her Seventies circumstances (loving film-star mother and neglectful, absent father). But then the entire movie is full of gaps and enigmas.
The film opens with a 10-minute sequence, set in Northern Iraq, which I find frankly baffling. The synopsis in the press kit tells me that it shows an ageing Jesuit confronting an ancient enemy, the demon Pazuzu, and Friedkin tells me the same thing in almost the same words. All I see is footage of sun; of desert; of an archaeological dig; of a small figure disinterred; of Max von Sydow looking gloomy (which comes with the territory); of a scary-looking statue. Is the idea that the evil is contained in the figure and follows the Jesuit back to America? Then how does it happen on Regan?
"I don't have to explain it. You're free to think of it, or to dismiss it, in any way that you want. It's called mystery. It's also raising certain questions that are not expected to be answered on the spot. Or in the next chapter." What I'm suggesting is that the absence of a crisp narrative may contribute to the film's perverse durability, but I have obviously said the wrong thing. Rather splendidly, Friedkin tells me that: "Jackie Collins writes crisp narrative. I suggest you read all of her books. You will never be in doubt about where the story is going."
William Friedkin doesn't seem to regard himself as an artist, but nor does he think of himself as a craftsman manipulating an audience. This is fascinating, since the audience consensus would have to be that The Exorcist is less a study in good and evil than "a very powerful and effective scare machine" (to borrow his description of Psycho, a film which to him has "no meaning"). Those polls of Best Horror Films aren't assessing theology.
Almost all of commercial cinema is a matter of manipulation, but that isn't an indictment. It can be the most obsessively manipulative film- makers (Hitchcock, Spielberg) who are able to rise above their own mastery. Doesn't he, too, work on a sequence until it has maximum impact? "I never start out thinking, `how can I hit 'em between the eyes?' I don't know how an audience will think. I don't think of an audience as being in the palm of my hand. The way I approach a scene is to make you believe that the people in the film are real. My only approach is to try to set a mood whereby the actors will be able to portray their characters realistically. No matter how well or poorly you craft the story, no matter how well or poorly you organise the images, the audience has its own operating structure."
So how does it feel that The Exorcist has such a durable impact? "I don't believe in old films, or old music, or old books." At first, this seems humble - that the culture must revitalise itself and move on - but it turns out to be a little less modest. "I believe there are works that speak to subsequent generations, and need not be called old." He mentions Bach cantatas, Vermeers, and in particular, Rubens' two versions of The Judgement of Paris.
And what films are timeless to him? "To me, the mark of a truly great film is that it universally makes people suspend disbelief. Some films do that for me - damn few. I don't believe one frame of The English Patient. I don't believe the people are real."
But in the case of a romance, isn't there a particular level of wish- fulfilment? When we watch Casablanca, we know this isn't the Second World War as fought... But I have trespassed on sacred ground, and Friedkin interrupts: "I suspend disbelief when I watch Casablanca."
Surely not to the extent of thinking they're real? "Yes, I think they're real. I believe in Rick and Ilsa. Yes. And the films I can still give myself to, a very hand-picked list" - a list that includes All About Eve and The Treasure of the Sierra Madre - "I can still be moved by emotionally. As a film director, that's what I try to do every time out, and I don't always, if often, succeed."