What do you write about a movie that pretty much created modern horror? Sure, you can point to Night of the Living Dead and even Carnival of Souls as starting points, but from a mainstream blockbuster perspective, this is where the rules of modern supernatural horror begin.

Inspired by William Peter Blatty’s 1971 novel, which itself was inspired by the 1949 exorcism of Roland Doe (“The Pope’s Exorcist” Malachi Martin claimed that he was the inspiration, a point that Blatty denied) the legends around this film — it was a cursed set, it’s filled with subliminal messages — supersede a very simple fact: this movie is frightening as hell, even 40 plus years later.

Do I even need to tell you the story of how Pazuzu finds its way into an Ouija board and into the soul of the daughter of an actress? Probably not. What’s striking is that how long the movie takes to get there. Scenes of Chris MacNeil (Ellen Burstyn) acting take precedence over the sad life of Father Karras, who has to deal with the death of his mother and his increasing lack of faith.

When do we realize something is wrong? When it is too late. When Regan (Linda Blair) intrudes on one of her mother’s boring parties, pisses on the floor and tells an astronaut “You’re gonna die up there.” PS — Want to know more about that guy? Then you should watch The Ninth Configuration.

Science can’t solve these issues. Detectives cannot. Only the Church can help.

What follows is a haunted house of scares that have been imitated ad nauseum (pun intended) so many times that we know the beats: head spinning, pea soup vomit, masturbation with a cross, blood, strange voices, levitation. A priest must show weakness before showing great sacrifice. And in the end, two old men find friendship in the aftermath.

It’s what is not seen that is most interesting, such as the old Hollywood directorial tricks William Friedkin used to get a reaction. He fired guns into the air to get a frightened reaction. He slapped some actors right across the face before important scenes. The painful screams of Blair and Burstyn are real — they were being yanked all over the set by stunt harnesses which caused both injuries and pain. And Regan’s bedroom was actually the inside of a freezer.

I’ve read for years about the subliminal that are supposedly hidden in the film, originally learning about them in William Poundstone’s book Big Secrets. Wilson Bryan Key — the guy who claimed that if you stack Ritz crackers up they always spell S-E-X — claimed that the film was full of images and sound effects that created a subliminal air of menace.

Friedkin has alternately claimed that subliminal messages are both “a very effective storytelling device. The subliminal editing in The Exorcist was done for dramatic effect — to create, achieve, and sustain a kind of dreamlike state” and that “there are no subliminal images. If you can see it, it’s not subliminal.”

One of the techniques used are the flashes of Captain Howdy, a demon who appears three times in the film. In an Entertainment Weekly article, Friedkin said, “You couldn’t catch it before VHS. And now you can stop the DVD and stare at it.” You’ll find the face during Regan’s examination, Dr. Karras’ dream, in the kitchen and several other places throughout the film.

Key also claimed that the word “pig” appears several times in the film, a keyword in the post-Manson era.

The site Subliminal Manipulation also writes that “the terrified squealing of pigs being slaughtered was mixed subtly into the soundtrack. The buzzing sound of angry, agitated bees wove in and out of scenes throughout the film.” They go even further to describe people fainting during the film and getting nightmares — attributed more to the subliminal than the horror content.  In addition, they say that “several theater employees were actually placed under the care of physicians and a few quit their jobs. Employees frequently had to clean up floors and rugs when nauseous spectators (mostly male, for some reason) did not quite make it to the restrooms.”

Want further conspiracy? That site also calls out author Blatty as a former CIA operative and policy-branch chief of the Psychological Warfare Division of the U.S. Air Force.

The music is another crucial element of the film. While Mike Oldfield’s Tubular Bells (actually part one of the overall record) is considered the theme for the movie, it was a complete accident that it was picked. Originally, Lalo Schifrin created six minutes of music for the trailer that were considered too frightening when combined with the images of the film. Some claim that this music was reused for The Amityville Horror, but the truth is that Friedkin hated the music that while it was being recorded, he made the orchestra stop playing and threw the tapes away in the studio parking lot — in front Schifrin and his wife. Additionally, the director claims that he wishes that had he heard them sooner, he would have used Tangerine Dream for the film (they would score his film Sorcerer).

While the film wasn’t available on video or TV in England until the 1990’s, an American TV cut has plenty of interesting changes, including director Friedkin speaking new, censored lines for Regan. In addition, this cut has a different shot of the Virgin Mary statue that cries blood and a longer shot of Regan’s face changing into a demon. This network TV cut is rarely seen today.

Friedkin used Mercedes McCambridge, a voiceover actress, to help create the signature sound of the demon’s voice. A former alcoholic, she used raw eggs, chain-smoking and whiskey to achieve the voice she used (and add to her state of mind). Plus, Friedkin demanded that she be bound to a chair while voicing the demon, to better give the sounds of restraint. She later said that the experience was one of horror and rage, while the director himself admitted that the extreme to which she went through terrified him. Plus, while she originally didn’t want any credit, once Linda Blair received a Best Supporting Actress nomination, she decided to sue so that people would know the voice as hers.

There’s also a Director’s Cut and an Extended Director’s Cut (The Version You’ve Never Seen) that has some cuts here and there, longer FX (CGI aided) for the spider walk scene and some added scenes between Father Karras and Father Merrin. These tweaks would only be noticeable to someone who has watched the film over and over again.

We’ll get to them eventually, but there were several sequels and prequels, such as Exorcist II: The Heretic, The Exorcist III, Exorcist: The Beginning and Dominion: Prequel to the Exorcist.

Was The Exorcist upsetting? Well, TV preacher evangelist Billy Graham claimed an actual demon was living inside the movie.

As for viewers today, they may be surprised at the sinister power that this movie still holds.