Though set in Washington, D.C., The Exorcist was partly shot in Manhattan at a studio on 54th Street. It was there that Linda Blair—as the possessed Regan MacNeil, now and forever—writhed and screamed and levitated on specially rigged beds. But when it came to that famous 360-degree head spin, a mechanical doll did the trick. And at some point, the doll’s designer, Marcel Vercoutere, took his life-size replica for a drive around town. If he noticed anyone gawking, he’d give the head a little spin.

This devilish detail comes from Nat Segaloff’s The Exorcist Legacy: 50 Years of Fear, a making-of compendium about the 1973 classic (and its less than classic sequels). If we imagine the reactions of Midtown office workers and Bergdorf lunchers to Vercoutere’s grisly companion, we might get closer to the shock value of seeing The Exorcist for the first time, before decades of imitations and references. Its mix of parent’s-nightmare psychodrama and taboo-busting horror quickly took hold, directed with focused intensity by the French Connection auteur William Friedkin.

The Exorcist basically minted the modern pop lexicon for demonic possession. (See the A24 summer scarefest Talk to Me.) But as a consummate “event movie,” it also came at the perfect time. If Rosemary’s Baby felt like a macabre lucid dream at the end of the 60s, The Exorcist tapped into deep fears about the uncertain future to come, when traditional beliefs might no longer protect us. Half a century later, it’s still an eye-opening watch. A bloody 12-year-old roaring “Let Jesus f*** you!”? A Marvel villain could never.

The Exorcist’s mix of parent’s-nightmare psychodrama and taboo-busting horror quickly took hold.

Segaloff, who wrote a 1990 biography of Friedkin, was the publicity director for a theater chain when The Exorcist came out, so he remembers the frenzy firsthand. But his new book dwells on production forensics: how the movie and its concept were developed and written and massaged (or manhandled, depending on who’s talking). Ellen Burstyn, who plays Regan’s mother, who is also an actress, says she “never thought of it as a horror film.” Segaloff seems to agree, recalling and refereeing the creative ping-pong over good and evil, faith and doubt, between Friedkin and William Peter Blatty, who wrote the best-selling novel and the screenplay.

Blatty had sought out Friedkin to direct, sending him the book manuscript before it was published. The Exorcist drew its authentic-sounding details from descriptions of an exorcism—the bed-shaking, the scratch marks, the subzero chill of the victim’s room. None of which apparently impressed Mike Nichols, Stanley Kubrick, or Arthur Penn, all of whom passed on directing. Friedkin clinched the job only after landing an undeniable hit with The French Connection. What followed, in Segaloff’s mostly heroic chronicle, was his exacting execution of an outrageous but effective vision (up to and including slipping an editor an audiotape of an exorcism ritual).

Thousands of girls auditioned to play Regan, but Linda Blair’s mother managed to get her daughter in front of Friedkin without an appointment. Friedkin must have appreciated the moxie—and the stability of the young actress, whose character would go through hell. Also crucial was casting Jason Miller as Father Karras, who takes on Regan’s case. Miller looked the part of a hapless, hangdog believer in crisis, holding his own opposite exorcist-in-chief Max von Sydow, one of Ingmar Bergman’s existential warriors.

Production ran to nine months, but whatever worries Warner execs had vanished after a rough-cut screening for top brass won Friedkin “carte blanche.” He’d trim the film further to hit a commercially appealing run time, thereby eliciting a lifetime of bellyaching from Blatty. But when it opened—the day after Christmas—the success meant The Exorcist was here to stay.

A bloody 12-year-old roaring “Let Jesus f*** you!”? A Marvel villain could never.

Sequels, prequels, a TV series—Segaloff’s accounting of Exorcist spin-offs and versions is positively Wikipedian in its thoroughness. But you get to read Louise Fletcher’s deadly line on starring in Exorcist II: The Heretic (“I didn’t have any deep frustration or questions except, ‘When is this movie going to end?’”), ponder Brad Dourif’s underrated talents (as a chilling serial killer in The Exorcist III—don’t ask), and flash back to pre-restoration Paul Schrader, who directed Dominion: Prequel to the Exorcist only to see someone else re-shoot most of it.

There’s only one original, however, and the litany of follow-ups—look out for a reboot by Halloween resurrector David Gordon Green—suggests we don’t want this demon out of our system anytime soon.

Nicolas Rapold is a New York–based writer and the former editor of Film Comment magazine