Quickly earning a name as the most terrifying film to grace the silver screen, the original Exorcist was unleashed onto cinema screens in 1973. Starring the wonderful Ellen Burstyn, Max von Sydown and Linda Blair, whose birthday we celebrate today, the film told the story a teenage girl who was possessed by a mysterious entity. However, the story off camera was just as interesting as the story presented to its audience. Here are eleven facts you never knew about the Exorcist... 1. Jason Miller is Jason Patric's father. Yes, this is a strange one but Jason Miller, who played the younger priest from the Exorcist, Father Damien Karras, is actually Jason Patric's father. We know, random. 2. The film is based on a true story. The film is based on the book The Exorcist, written by William Peter Blatty, which was based on a real exorcism. In 1949, Ronald Hunkeler was sent to a relative's home in St. Louis for a real exorcism. 3. The vomiting scene took only one take. Apparently, the vomit was supposed to hit Karras in the chest but the plastic tubing that sprayed the vomit misfired. His look of shock was real and never edited. 4. Kubrick turned the Exorcist down. Yes, that Kubrick. Stanley Kubrick. Imagine. 5. Jason Miller studied to be a priest. Miller, who played the younger Karras, studied to be a priest in University of Washington. Apparently, he dropped out after three years because he "lost his faith." 6. The Head Spinning Scene was almost cut out. The Spider Walk got taken out until the director's cut, but imagine if the head-spinning scene was removed. Blatty had originally asked Friedkin to remove it. 7. There was nothing Mercedes McCambridge would not do to get that voice down... Mercedes provided the voice of the demon, which she apparently achieved by eating raw eggs and chain smoking. She also had herself tied to a chair with pieces of torn sheet around her neck to achieve the vocal effect. McCambridge was never credited for her performance. 8. There was a fire on set. But wait, the fire burned all of the set... except Regan's bedroom! 9. Friedkin actually hit the priest during the last scene. The priest who gave the last rights in the Exorcist was actually shaking in the scene. Rumour has it that Friedkin actually hit the man before the take, leaving him shaking uncontrollably. 10. The lead actresses were both injured. Both of the lead actresses on the film were injured quite badly during filming, Blair from the scene where her body is thrown back and forth on the bed and Burstyn from when she was pushed away on entering the room. 11. The set was constantly blessed. The cast and crew were so convinced they were working on a cursed project, particularly after some of them lost their lives, that Blatty and Friedkin regularly asked for a priest, Father King, to come in and bless the set.