You may have forgotten the patient, slow-burning first act of William Friedkin’s The Exorcist. It’s all a ploy, a peaceful and welcoming slice of life luring you into a false sense of security. We meet Father Merrin (Max von Sydow), exploring the sands of Northern Iraq, discovering sinister idols. Then we hop over to Georgetown, and are invited into the home of actress Chris MacNeil (Ellen Burstyn) and her sweet young daughter, Regan (Linda Blair). Regan is purposefully normal, innocent and open faced, and we feel the love between her and her famous but attentive mother.

We meet Father Karras (Jason Miller), a priest struggling with his faith, concerned with the well-being of his dying mother. The moments with his mother, and these quiet moments with Chris and Regan, are pleasant glimpses of humanity. Friedkin’s cameras observant and caring, and these early scenes have an almost Altman-esque drift as we casually observe conversations. But there’s that strange scratching noise coming from the attic of Chris’s brownstone. And Regan seems to be acting unusual.

Then, all hell breaks loose.

The Exorcist unleashes pure insanity in ways most films and filmmakers would never dare. It is a full-frontal assault on the humanity built up in this calming first act. Regan’s possession steadily increases, from swearing binges, to body attacks, to cuts and scrapes and voices gurgling from deep inside her body. All the while, doctors tell Chris it’s a neurological problem, a blood clot in her temporal lobe. When that proves false the doctors point Chris the way of a psychiatrist. Nobody wants to even consider such an outlandish idea of a demonic possession. That is one of the core strengths of The Exorcist, its dedication to reality. This is the real world here, demonic possessions are ridiculous and antiquated studies. Even when Chris approaches Father Karras, his reluctance to call Regan’s affliction a demonic possession intensify what the audience knows; this is the work of Satan, and why will nobody admit it?

That is the true horror at the heart of the film: Desperation. As Regan falls deeper and deeper under the control of the demon inside her, Chris and the doctors, and even Father Karras, are seemingly helpless. Much has been made over the years of Linda Blair’s disturbing, effective performance as she becomes this demon and all signs of poor Regan disappear. But perhaps the more unsettling performance is that of Burstyn as Chris. Regan becomes the demon, any semblance of a 12-year old girl vanishing beneath the brilliant makeup. She is the monster of this monster film, leaving her mother helpless and desperate beyond belief. Burstyn’s performance is unhinged and nerve-frying. She cries out continually for someone, anyone, to help her and help her daughter. Her begs are severe, high-pitched, frustrated. It is a draining performance by Burstyn, who gradually slips into her own sort of sleep-depraved madness in the midst of the unexplainable. Burstyn’s Chris is the representation of the audience, who themselves grow increasingly desperate in the face of such insanity.

Even the arrival of Father Merrin creates unease, as Friedkin shoots von Sydow’s imposing frame to seem even bigger, broader, and more dominating, a monster to fight a monster. There is no release in this film, even once Regan is free of the demon and Father Karras hurls himself through the window and down the stairwell.

William Friedkin is a director who never shies away from dragging the audience through the intense gauntlet of the most horrific imagery. Here, Friedkin throws everything relentlessly at the screen. Head spinning, crucifix-masturbating, vomiting, Regan is no longer an innocent young girl; she is a vehicle for Satan. The “true” story which served as the inspiration for William Peter Blatty’s novel centered on a boy, not a girl. Friedkin’s decision to change the subject to a girl was primarily to protect the players in the real story. But it also works to enhance the desperation; a young girl brings with it more vulnerability to exploit. The Exorcist opens the floodgates of disturbing horror unlike any film, and is so effective because the subject of the film is not an adult corrupted by society, but an innocent young girl.