People have been arguing about Invasion of the Body Snatchers’ message for 60 years now. Some believe it’s a coded warning about the spread of communism. Others—like novelist Jack Finney (upon whose story the movie is based)—see it as a thoroughly non-political thriller. Regardless, fans of all stripes agree that Invasion of the Body Snatchers is an enduring horror classic. Released on this day in 1956, it still sends shivers up the spine.

1. IT’S BASED ON A MAGAZINE SERIAL.

In November and December of 1954, Collier’s magazine ran a three-part series that would come to be called “the year’s most original story of suspense.” Written by Jack Finney, The Body Snatchers wowed producer Walter Wanger, who began negotiating the story’s movie rights before he’d even read part two.

2. IT WAS SHOT IN JUST 23 DAYS.

With a modest $380,000 budget (roughly $3.3 million in today’s dollars), Invasion of the Body Snatchers started filming in Sierra Madre, California on March 23, 1955. If you’re a horror buff, the little city may look a bit familiar, since segments of Halloween (1978) and The Fog (1980) were shot there as well.

3. DIRECTOR DON SIEGEL PRANKED HIS LEAD ACTRESS WITH ONE OF THE FAKE PODS.

The film’s heroine, Becky Driscoll, was played by Dana Wynter. Rumor has it that Siegel once broke into Wynter’s home and hid a human-sized pod prop under her bed, though the actress remembered the incident differently. “He left it on my doorstep,” Wynter recalled in a 2001 interview. “Don Siegel was courting this girl [who lived next door], and he would pass my cottage all the time. And one night, he just left it on the doorstep ... I nearly broke my neck, because when you open your front door to go to your car, you don’t expect to find something large on your doorstep.”

4. THE LEADING MAN SUGGESTED A TITLE CHANGE.

In the film, alien pods replace slumbering people with emotionless duplicates. Hence, when Kevin McCarthy landed the lead role of Dr. Miles Bennell (who discovers what’s really going on), he suggested that Invasion of the Body Snatchers should be renamed Sleep No More.

You might be wondering why the final title differs from the one that Finney’s original series went by. In 1884, Robert Louis Stevenson had written a short story called The Body Snatcher, which was turned into an RKO film in 1945. To avoid confusion with that earlier movie, Siegel’s flick was rebranded Invasion of the Body Snatchers.

5. ORIGINALLY, THE MOVIE WAS SUPPOSED TO BE A LOT FUNNIER.

“I felt that pods growing into a likeness of a person would strike the characters as preposterous,” Siegel recalled. “I wanted to play it that way, with the characters not taking the threat seriously.” Hoping to offset the scares, he filmed a number of comedic scenes, which were later cut out by Allied Artists, the film’s distributor. “In their hallowed words, ‘horror films are horror films and there’s no room for humor,’” Siegel explained. “I translated [this] to mean that in their pod brains there was no room for humor.”

6. FULL-BODY MOLDS OF THE LEAD ACTORS WERE BUILT.

In the creepiest scene in the movie, the four main characters discover half-formed clones of themselves emerging from a quartet of pods. To create these facsimiles, McCarthy, Wynter, King Donovan (“Jack Belicec”), and Carolyn Jones (“Teddy Belicec”) were laid down on slanted boards, where the crew coated them, from head to toe, in plaster of Paris. Subsequently, these molds were filled with foam rubber. During the lengthy process, Wynter was subjected to yet another practical joke—this time, courtesy of the mold-builders. “I was in this thing while it hardened,” she said. “I was breathing through straws … and the rest of me was encased, it was like a sarcophagus. The guys who were making it tapped on the back of the thing and said ‘Dana, listen, we won’t be long, we’re just going out for lunch!’”

7. THE FILMMAKERS FEARED THAT KEVIN MCCARTHY MIGHT NOT SURVIVE THE ENDING.

The film’s action required leading man Kevin McCarthy to run for days on end. In numerous scenes, his character sprints for dear life over every possible terrain. “I got Charlie horses,” admitted McCarthy. Just before the film draws to a close, Dr. Bennell runs through traffic in a panicked frenzy, screaming “They’re here already! You’re next! You’re next!” Since the exhausted actor hadn’t been sleeping well, Siegel told his stunt drivers to remain extra alert in case McCarthy tripped without warning. “I was terrified that his timing would be off and he might fall down under the wheel of the cars and trucks,” Siegel admitted.

8. THE PROLOGUE AND EPILOGUE WERE LAST-MINUTE ADDITIONS.

Allied Artists didn’t just cut a few jokes here and there; the studio also insisted on a completely different ending. Originally, the movie was going to close with a shot of Dr. Bennell watching hopelessly as truckloads of pods drive out into the distance. Wanting to end the film on a more hopeful note, Allied Artists came up with a slightly happier conclusion. Over his strong objections, Siegel was told to film a new intro and a new final scene (“I reluctantly consented,” he said.) The revamped opening puts Bennell in a police station, where he tells the story as an extended flashback. After the famous “You’re next!” sequence, his tale ends and, after a while, the authorities begin to believe him.

9. IT COULD’VE BEEN NARRATED BY ORSON WELLES. OR RAY BRADBURY.

Walter Wanger desperately wanted two Orson Welles scenes to bookend the movie. As a prologue, the Citizen Kane director would offer an unsettling soliloquy. Then, at the end of the movie, he’d return with this sobering advice: “In this day and age, anything can happen. And if you’re asleep when it does, you’re next.” Unfortunately, scheduling conflicts kept Welles out of the film. Wanger later toyed with the idea of giving Welles' part to legendary sci-fi author Ray Bradbury, but ultimately opted to cut out the narrator role altogether.

10. MCCARTHY MADE A CAMEO IN THE 1978 REMAKE.

Donald Sutherland assumed leading man duties for Philip Kaufman’s 1978 remake of the film, which was a gorier and gloomier take on the story. At one point, two generations cross when Kevin McCarthy hurls himself at the younger actor’s car and screams “Help! They’re coming! Listen to me!”