Milos Forman's One Flew Over the Cuckoo’s Nest, which premiered on this day in 1975, won critical acclaim, box office success, and a shelf full of Oscars. But even if you love the complex exploration of life inside a 1960s psychiatric hospital, there are a few things you may not know about its behind-the-scenes story.

1. CUSTOMS NEARLY DOOMED THE PROJECT.

Despite the middling success of the 1963 stage adaptation of Ken Kesey’s novel starring Kirk Douglas, Hollywood legend Douglas was dead set on adapting the story for the screen. Douglas contacted Czech director Miloš Forman about the project, promising to send Forman a copy of the book for his perusal.

Douglas mailed Forman the novel, but the package was confiscated by Czechoslovakian customs and never reached the director. Unaware of the parcel’s fate, the filmmaker resented Douglas’s broken promise, and Douglas thought Forman rude for never bothering to confirm receipt of the novel. It took a decade to sort the mess out, and things only cleared up when Kirk’s son Michael Douglas took another crack at production and contacted Forman once more.

2. ONE STUDIO WANTED TO CHANGE THE ENDING.

When producers were shopping the picture to studios, 20th Century Fox was interested, but with a catch. Fox would distribute the film, but only if the filmmakers would agree to rewrite the ending; the studio wanted McMurphy to live. Producers Saul Zaentz and Michael Douglas wisely considered this a deal breaker, and United Artists eventually distributed the film.

3. JACK NICHOLSON AND LOUISE FLETCHER WERE NOT THE FIRST CHOICES FOR THEIR CHARACTERS.

Warner Bros.

When Kirk Douglas spearheaded the first attempt to bring One Flew Over the Cuckoo’s Nest to life on the big screen in the 1960s, he had intended to play the Randle Patrick McMurphy role himself, just as he had on stage. When production began in earnest 10 years later, Douglas was too old for the part, leaving director Forman to consider and contact the likes of Gene Hackman, Marlon Brando, and (his personal favorite) Burt Reynolds before finally settling on Jack Nicholson.

A number of different actresses were considered for the role of Nurse Ratched, the film’s central antagonist, as well: Anne Bancroft, Colleen Dewhurst, Geraldine Page, and Angela Lansbury were all in the running, before Louise Fletcher ultimately got the part.

4. LOUISE FLETCHER CHANGED FORMAN’S VIEW ON THE CHARACTER.

Forman’s original view of Nurse Ratched was as “the personification of evil,” a characterization that made Louise Fletcher a bad fit for the part in the filmmaker’s mind. As Fletcher pressed for the role, Forman’s perspective of Ratched evolved: “I slowly started to realize that it would be much more powerful if it’s not this visible evil,” he said. “That she’s only an instrument of evil. She doesn’t know that she’s evil. She, as a matter of fact, believes that she’s helping people.” This new take on the character paved the way for the official casting of Fletcher.

5. SEVERAL OF THE FILM’S STARS WERE NOT ACTORS.

Following the production team’s decision to use Oregon State Hospital as its shooting location, the producers hit on the idea of casting facility superintendent Dr. Dean Brooks as Dr. John Spivey, the doctor charged with assessing R. P. McMurphy’s psychological health. Brooks agreed to play what turned out to be a sizable role, though it would be the only acting job he would ever take. He also helped secure employment for many of his hospital’s patients as extras and crew members during production.

Mel Lambert, another non-actor, was wrangled to play the harbormaster who protested McMurphy’s ad hoc fishing trip. What’s more, Lambert—a respected area businessman who had a strong relationship with the local Native American community—introduced the production team to Will Sampson, the 6-foot-5-inch-tall Muscogee painter who would make his acting debut as the major character Chief Bromden.

6. THE STARS LIVED ON THE WARD DURING PRODUCTION.

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All of the actors who played patients actually lived on the Oregon State Hospital psychiatric ward throughout production. The men personalized their sleeping quarters, spent their days on campus “get[ting] a sense of what it was to be hospitalized” (as actor Vincent Schiavelli put it), and interacting with real psychiatric patients.

7. MANY SCENES WERE SHOT WITHOUT THE ACTORS’ KNOWLEDGE.

To complete this realistic immersion, Forman led his performers in unscripted group therapy sessions in which he directed the actors to develop their characters’ psychological maladies organically. He would often capture footage of the actors, both in and out of character, without explicitly mentioning that the cameras were rolling. The film’s final cut includes a shot of a visibly irritated Fletcher reacting to a piece of direction fed to her by Forman.

8. FORMAN AND NICHOLSON HAD A TREMENDOUS SPAT OVER THE FILM’S PLOT.

While the intensity of the turmoil varies from rumor to rumor, reports from the set were consistent on one fact: The star refused to speak with Forman for a large chunk of the production process. Nicholson took issue with Forman’s suggestion that the hospital inmates would be an unruly bunch upon the initial arrival of McMurphy. Instead, the actor insisted that such disavowal of the medical staff’s authority should only begin after the introduction of McMurphy into their lives and routines.

Although the version of the story that we see in the film today is more closely associated with Nicholson’s alleged reading, suggesting that Forman ultimately took his advice, Nicholson refused to interact with his director from that point forward. When the star and Forman needed to communicate with one another, they used cinematographer Bill Butler as a middleman.

9. DANNY DEVITO CREATED AN IMAGINARY FRIEND DURING PRODUCTION.

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Emotionally strained by a demanding shooting schedule that kept him 3000 miles from his future wife, Rhea Perlman, DeVito developed the coping mechanism of an imaginary friend with whom he would have nightly chats. Concerned that his own sanity might be slipping away, DeVito sought the advice of Dr. Brooks, who assured him that there was no reason to worry as long as DeVito could still identify the character as fictional.

10. THE CREW WAS WORRIED ABOUT THE SANITY OF ONE CAST MEMBER.

While Dr. Brooks had no concerns about DeVito, he echoed the rest of the cast and crew’s apprehensions about the psychological state of Sydney Lassick, who played Charlie Cheswick. Lassick exhibited increasingly unpredictable and emotionally erratic behavior during his time in character, a pattern that culminated in a tearful outburst during his observation of the final scene between Nicholson and Sampson. Lassick became so overwhelmed during the scene that he had to be removed from set.

11. FLETCHER TOOK OFF HER CLOTHES IN ORDER TO GET FRIENDLIER WITH HER CO-STARS.

Envious of the camaraderie her male costars had forged, and hoping to dispel any associations with her tyrannical character, Fletcher surprised the cast one evening by ripping off her dress on the crowded ward. Years later, the actress laughed about the display, saying, “‘I’ll show them I’m a real woman under here, you know.’ I think that must have been what I was thinking.”

12. THE FISHING TRIP SCENE BARELY MADE IT INTO THE FILM.

Initially, Forman was vocally opposed to including a scene that took place beyond the grounds of the hospital out of concerns that a temporary liberation would undercut the dramatic force of the film’s ending. In the end, Zaentz convinced Forman to shoot the fishing trip sequence. It was the final scene filmed and the only piece shot out of chronological order.

One thing to look for in the fishing scene: A very subtle Anjelica Huston cameo. Huston, who was dating Nicholson during production, has a nonspeaking role as one of the spectators on the dock as McMurphy and his fellow patients steer the stolen boat back to shore.

Warner Bros.

13. ONE FLEW OVER THE CUCKOO’S NEST WAS THE FIRST FILM TO WIN ALL “BIG FIVE” ACADEMY AWARDS IN 41 YEARS.

Not since 1934's It Happened One Night swept the Oscars had a film walked away with awards for Best Picture, Best Director, Best Actor, Best Actress, and Best Screenplay. One Flew Over the Cuckoo’s Nest took home the lot, with Nicholson and Fletcher winning the top acting awards. The feat would not be matched again for another 16 years, with Silence of the Lambs becoming the next (and last to date) movie to earn the distinction.

14. THE FILM ENJOYED ONE OF THE LONGEST THEATRICAL RUNS IN MOVIE HISTORY.

One Flew Over the Cuckoo’s Nest was revered worldwide, but Swedish viewers developed an especially soft spot for the film. Cuckoo’s Nest remained a regular option for Swedish moviegoers through 1987—11 years after its initial release.

15. KESEY REFUSED TO SEE THE FILM (BUT MAY HAVE BY ACCIDENT).

The poster child for the “the book was better” movement, One Flew Over the Cuckoo’s Nest author Kesey disapproved of a big screen adaptation of his novel as soon as he found out that the filmmakers had abandoned the use of Chief Bromden as the story’s narrator. Kesey never intended to see the movie, but one story says he inadvertently caught a few moments during a bout of channel surfing one evening. Once Kesey realized what he was watching, he promptly changed stations.

According to fellow novelist Chuck Palahniuk (who has famously praised director David Fincher’s adaptation of his novel Fight Club, plot changes and all), Kesey once stated privately that he did not care for the material.