Since its initial theatrical release 45 years ago, on December 26, 1973, The Exorcist has remained among the highest grossing horror movies of all time. The first horror movie to be nominated for Best Picture at the Academy Awards, this seminal classic fundamentally altered the genre irrevocably. Centered on a possessed teen girl and a priest suffering a crisis of faith, The Exorcist became the standard by which most horror films that followed were judged, in terms of scare factor, anyway. Not only did every possession film that came after draw inspiration from this film, but it spawned multiple sequels, prequels, television series, theatrical re-releases and more in its four-decade-long legacy. The original, true-story exorcism story that inspired author William Peter Blatty’s novel, tales of a curse plaguing the set of the film, and many other details surrounding The Exorcist have long been uncovered already. But here are 10 things you might not know about this horror classic:

Rosemary’s Baby paved the way for The Exorcist

Having embraced religion from a young age, and deeply inspired by the 1949 case about a possessed boy in Mount Rainier, Maryland since his college days, author William Peter Blatty really wanted to write a story inspired by the exorcism. Most mainstream movies tended to lean heavily for or against religion, though, making Blatty feel as though what he sought to do with his story would never fit mainstream ideals. However, when he saw Rosemary’s Baby in the theater he was inspired by the way the film turned the ambiguity and questions of faith and belief into a mainstream film. It was the push he needed, and he wrote the novel in a mere 10 months.

William Friedkin got the job because of William Peter Blatty

UNSPECIFIED – CIRCA 1974: Linda Blair and William Friedkin On The Exorcist Shooting, In 1974 (Photo by Keystone-France/Gamma-Keystone via Getty Images)

By now it’s likely well known that Friedkin wasn’t the first choice to direct. The studio approached the likes of Stanley Kubrick and Mike Nichols first, who both turned the project down. Then the studio eyed Mark Rydell. Blatty, however, pushed for Friedkin. Between Friedkin’s work on hit The French Connection, and Blatty’s assertiveness in getting his way, well, Friedkin got the job in large part to the screenwriter.

William Peter Blatty played dirty to get his way

Typically, screenwriters don’t get so intertwined with the film. They write, and then it’s handed over from there. But no one was as invested in their story as Blatty, who was so passionate about The Exorcist and getting it correct. In Jason Zinoman’s novel Shock Value, it was revealed that Blatty actually snuck into producer Paul Monash’s office and went through his personal files until he found a paper trail revealing a deal with Warner Brothers that involved a completely restructured script. Blatty took that to his agent, who then went to Warner Brothers’ vice president, claiming false representation. Monash was removed from the project, and Blatty then became producer instead, his story once again intact.

Mercedes McCambridge provided the voice of Pazuzu

One of the most riveting performances in the film (among many stellar performances) is also one of the most unheralded. The demonic, possessed voice of Regan MacNeil (Linda Blair) was performed by radio actor Mercedes McCambridge. Her radio acting skillset combined with her distinct voice, further made husky by insistence on drinking whiskey, chain-smoking, and swallowing raw eggs contributed to a terrifying performance. Having suffered from chronic bronchitis, McCambridge cleverly used her wheezing to create the bone-chilling breathing of Pazuzu. For a long while, McCambridge didn’t receive credit for her voiceover work; she didn’t want to take away from Blair’s performance according to Friedkin.

Father Damien Karras was Jason Miller’s first film role

After earning degrees in English and philosophy, Miller worked various odd jobs like welfare investigator, waiter, truck driver and messenger boy while writing plays. While he ultimately won a Pulitzer Prize for his play The Championship Season, the income wasn’t steady and he gave up professional writing for acting instead. The year he won his Pulitzer was the same in which he was offered the role of Father Damien Karras, the priest struggling with his faith. That role, his first in film, earned him an Academy Award nomination for Best Supporting Actor.

The projectile vomit scene took only one take

It’s now well known that the green vomit spewed by the possessed Regan was actually made up of pea soup. What’s not as well-known is that Miller’s horrified and disgusted reaction isn’t acting but genuine. Because it was real, the shot was nailed on one take. Originally, Regan’s projectile vomit was supposed to land squarely on Miller’s chest, but the tubing hose misfired, catching Miller off guard. It was a technical error that made Miller angry, he later confessed in interviews.

Max von Sydow went through a lengthy aging makeup process for his role

Father Merrin, the experienced veteran Catholic priest selected to perform the exorcism of 12-year-old Regan, is a 79-year-old with a weakened heart. At the time of filming, von Sydow was only 43. The actor underwent a 3-4 hour makeup application process every day to resemble the aged priest. Between the stellar makeup effects and von Sydow’s amazing portrayal of Merrin, it’s easy to forget how young the actor really was at the time.

Blatty hates the movie’s ending

Different views on how to handle the story meant a lot of tension between Friedkin and Blatty over the course of production. For Friedkin, he wanted a much more ambiguous story, one that left the viewer questioning the mysterious nature of the possession and the motivations. Blatty’s story, however, was much more explicit in its meaning. His novel spells out exactly why Regan was chosen and what Karras’ sacrifice meant. Friedkin’s ending made Blatty feel it would be interpreted that the Devil won, which was in extreme opposition to the point of his story.

The original theatrical release caused “cinematic neurosis”

Over the holiday weekend that The Exorcist opened and the weeks that followed, visceral audience reactions swept the country. Fainting, vomiting, panic, and even reports of miscarriages and heart attacks lead to further reporting of panic and hysteria surrounding the film’s release. The frenzy was so publicized that it even lead to medical journalists giving a psychiatric name for the craze associated with the horror film, titled “cinematic neurosis.” To say that the film struck a nervous chord in America would be putting it mildly.

The Exorcist set the trend for the inevitable horror sequel

Warner Brothers didn’t think the film would be a huge hit. They didn’t bother to screen it for critics and they only put it out in 30 theaters at first. They were proved wrong right away, when the film caught on like wildfire, quickly become the must-see movie of the year. Wanting to capitalize further on its success, Warner Brothers jumped on the opportunity to continue the story of Regan and Pazuzu with The Exorcist II, opening in 1977. While this sequel was panned and nowhere near as successful as its predecessor, Warner Brothers essentially pioneered the horror sequel as we know it today. Major horror films before, like Night of the Living Dead or Rosemary’s Baby hadn’t had any follow ups. The Exorcist II paved the way for the standard practice of the horror sequel.