CONTENT WARNING: I discuss some psychological case studies with disturbing details. Plus, vomit.
Are people so numb they need movies of this intensity in order to feel anything at all?
-- From Roger Ebert's 1973 review of The Exorcist
The reactions do seem to be real; at the movie's release, Time interviewed a theater manager that estimated an average of "four blackouts and six episodes of vomiting" per screening, and another manager said "my janitors are going bananas wiping up the vomit". A review noted "you couldn't even get near the sink" of the bathroom after the show.
A video that went viral back in 2014 collected news reactions of audience members; however, there's editing focused on the more extreme reactions. A newscast from the WFAA Newsfilm Collection gives a better "random person" collection but even that one includes someone who had to leave the theater early.
Warner Brothers was not above ginning up controversy for publicity, and even claimed the movie received condemnation by the Catholic Church (this was not true). However, there were too many reports from too many corners to say the reactions were all manufactured.
As far as why this happened, some of this is likely joint audience reaction. The New York Times mentioned four hours or more of a wait to get inside the theater, and once the movie was rolling
During the exorcism, there was continuous screaming in the theater and it sounded like the old screaming‐for‐screaming's‐sake that one used to hear at early Beatles and Rolling Stone concerts.
which, rather like laughter at a comedy, can only serve to amplify the effect.
It's also important that director William Friedkin obsessed over the technical effects to create a seamless experience. The bedroom scenes were filmed in an actual freezer to make the room genuinely cold, rather than having the actors pretend. The "spider" deleted scene which made it to the Director's Cut was removed because the wires were too visible in 1973. Despite the audience already being familiar with horror classics like Psycho, technical capabilities were only just reaching the point in 1973 to accomplish special effects in a naturalistic way. (Compare with Rosemary's Baby from 1968 -- while a landmark in horror, the scene involving the Devil was odd and dreamlike.)
1973 was also part of a wave of Satan that had started in the 1960s. John Hagee, megachurch pastor, wrote the 1973 book Invasion of Demons: The Battle Between God and Satan in Our Time describing "occult practices sweeping America in a satanic revival". Things crested starting in the 1980s with the Satanic Panic, but that's a much longer topic for a different time. Suffice it to say: the movie (and book) were advertised as "based on a true story" and there were people who believed possession by the devil could actually happen.
Still, even given all these things, The Exorcist exceeded all prior norms on audience reaction, so let's consider the most extreme reactions: the people who had to get psychological help after watching.
The psychiatrist Ralph Greenson (in a 1974 lecture) described a patient who would "rave, rant, cry, beg, stamp his feet, and bang his fists" and stated he had a "a devil" inside him, and he was convinced of it from watching The Exorcist. Dr. Greenson inquired of his colleagues if any them also had patients with similar reactions; all of them did.
From The Journal of Nervous and Mental Disease the doctor James Bozzuto described four case studies of "traumatic neurosis".
Case 1 involved an 18 year old male who had insomnia for 3 weeks. He had, during military service, joined a Bible study group and "asked God into [his] heart" 2 weeks before his first leave. During that leave, he saw The Exorcist with friends, and the first night after was unable to sleep and was found by his father clutching his Bible at 4 or 5 am. He was worried he was in danger of possession by the devil, and started abusing drugs in order to purge the memory of the movie.
Case 2 was a 23 year old male referred by a priest. He knew about the movie before watching it (with his wife) but the next evening while his wife and 4-year old daughter were out he started to get the feeling of being possessed, as well as thinking his wife was about to suffer some sort of injury or accident. In the following weeks he had "persistent magical dreams" of the Devil and, in his own words, "I entertained the idea I could be guilty about something."
Case 3 was a 22 year old female who had difficulty sleeping after watching to movie. She had went to the movie with her boyfriend and was worried if the young priest in the movie had gone to Hell. She became preoccupied with "the Devil aspect" and evil forces. She had a Catholic upbringing but was now living with her boyfriend "in sin" and felt the movie brought up "guilt of punishment" and was afraid of "losing control".
Case 4 was a 24 year old male with two children who saw the movie with his wife and a second couple. He became upset during the movie and had to leave halfway, worried that the Devil "would come." He got insomnia and started to worry his 5-year old was possessed, and after a visit at a Psychiatric Clinic, was concerned that the therapist he had met was in league with the Devil.
Note that all these case studies have religiosity in common. Bozzuto points out the movie is themed around "loss of impulse control" and theorized that the people who are close to the "stimulus barrier" fell into mental illness, calling the phenomenon "cinematic neurosis".
Curiously, the one element that did not seem to cause the stress issue was the movie's violence. Departing from case studies, a 1980 psychology study with volunteers used The Exorcist and Jaws to test a number of hypotheses in terms of "stressors", and found the top 4 stressor elements were
Intensity of Film
Showed people being used, abused, or victimized
Gloomy or depressing mood
Intensive negative outlook or theme
These describe The Exorcist amply, including the "showing people being used, abused, or victimized". This is partly born out by the fact that -- based off of walk-out times and specific testimony -- one of the most upsetting scenes is not the possession antics at the end, but the tests of the girl at the hospital in the first half.
The doctors’ probing of Regan’s body appears ultimately to be as crude and as barbaric as medieval bloodletting...
Putting everything together:
a.) the long waits in line and stories about fainting, vomiting and so forth served to increase the tension of moviegoers before they even went inside
b.) the theater itself had "continuous screaming" which amplified the effect
c.) the special effects were carefully curated to avoid breaking the feeling of reality; technology had reached a point by 1973 where this was possible
d.) a number of people who believed the "true story" aspect to the possession and especially the idea that real demons were spreading
e.) the story was not one which required a random killer; it presented the possibility of loss of control by possession as something that could happen suddenly, frighteningly, and without rational cause
f.) the "primary stressors" of moviegoers are emphasized, making the victim's helplessness resonate stronger with the viewers
...
Bozzuto, J. C. (1975). Cinematic neurosis following "The Exorcist": Report of four cases. Journal of nervous and mental disease.
Chambers, A. C. (2021). ‘Somewhere between science and superstition’: Religious outrage, horrific science, and The Exorcist (1973). History of the Human Sciences.
Greenson, R. (2018). On Loving, Hating, and Living Well: The Public Psychoanalytic Lectures of Ralph R. Greenson. United Kingdom: Taylor & Francis.
Hamilton, J. W. (1978). Cinematic neurosis: A brief case report. Journal of the American Academy of Psychoanalysis, 6(4), 569-572.
Johnson, B. R. (1980). General Occurrence of Stressful Reactions to Commercial Motion Pictures and Elements in Films Subjectively Identified as Stressors. Psychological Reports, 47(3), 775–786.
Laycock, J. (2009). The Folk Piety of William Peter Blatty: "The Exorcist" in the Context of Secularization. Interdisciplinary Journal of Research on Religion, 5.
Woodward, K.L. (11 Feb 1974). The exorcism frenzy. Newsweek, 60-66.