In the summer of 1973, newbie director Tobe Hooper—who passed away on August 26, 2017 at the age of 74—and a group of unknown actors ventured out into the Central Texas heat to make a horror movie. Braving blistering temperatures, on-set injuries, and a shoestring budget, they produced one of the most terrifying motion pictures ever made.

More than four decades after its release, The Texas Chainsaw Massacre still shocks and thrills audiences with its realistic imagery, unhinged tone, and “based on a true story” marketing—and its status as one of the ultimate cult classics shows no signs of fading. Not bad for a little film that drove the cast and crew insane during production. From marathon shooting days to flying chainsaws to mafia money problems, here are 20 facts about one of the greatest slasher films of all time.

1. IT WAS INSPIRED BY A CHRISTMAS SHOPPING CROWD.

The inspirations for The Texas Chainsaw Massacre are surprisingly diverse, ranging from director and co-writer Tobe Hooper’s attempt to make a modern retelling of Hansel and Gretel to real-life Wisconsin murderer and corpse defiler Ed Gein. According to Hooper, though, the light bulb moment that really ignited the film came at a department store during the Christmas 1972 shopping rush.

"There were these big Christmas crowds, I was frustrated, and I found myself near a display rack of chain saws. I just kind of zoned in on it,” Hooper told Texas Monthly. “I did a rack focus to the saws, and I thought, ‘I know a way I could get through this crowd really quickly.’ I went home, sat down, all the channels just tuned in, the zeitgeist blew through, and the whole damn story came to me in what seemed like about 30 seconds. The hitchhiker, the older brother at the gas station, the girl escaping twice, the dinner sequence, people out in the country out of gas.”

2. LEATHERFACE IS ALLEGEDLY BASED ON A REAL PERSON HOOPER KNEW.

Leatherface, the chainsaw-wielding maniac who would go down in history as one of horror cinema’s greatest villains, shows obvious Ed Gein influence thanks to his mask crafted from human skin, but Gein was not the character’s only precursor. The idea of a mask made of human skin actually came to Hooper far more directly, and creepily.

“Before I came up with the chainsaw,” Hooper said, “the story had trolls under a bridge. We changed that to the character who eventually became Leatherface. The idea actually came from a doctor I knew. I remembered that he’d once told me this story about how, when he was a pre-med student, the class was studying cadavers. And he went into the morgue and skinned a cadaver and made a mask for Halloween. We decided Leatherface would have a different human-skin mask to fit each of his moods.”

3. THE TEXAS CHAINSAW MASSACRE WAS NOT THE ORIGINAL TITLE.

After inspiration struck, Hooper and co-writer Kim Henkel hammered out a script over several weeks and gave it the eerie title Head Cheese (named for the scene in which the hitchhiker details the process of how that particular pork product is made). Then it was changed to the menacing working title of Leatherface. It wasn’t until a week before shooting was set to begin that the eventual title arrived, suggested to Hooper and Henkel by Warren Skaaren, then head of the Texas Film Commission, who’d helped the project get financing.

4. IT IS NOT A TRUE STORY.

Though the real crimes of Ed Gein did influence Hooper and Henkel in their writing, the idea that The Texas Chainsaw Massacre is itself based on a true story is something that grew out of the marketing of the film. The opening narration, which promised that “The film which you are about to see is an account of the tragedy which befell a group of five youths,” certainly helped that along, as did the original poster and its promise that “what happened is true!” Despite this clever aura, the tale of Leatherface and his deranged family is still a work of fiction, despite continued protestations from fans even decades later.

“I’ve had people say ‘I knew the original Leatherface,’” Gunnar Hansen, who played the killer character, recalled.

5. GUNNAR HANSEN WAS NOT THE ORIGINAL LEATHERFACE.

It’s hard to imagine anyone but the massive Gunnar Hansen, who passed away in 2015, behind the Leatherface mask in the original film now, but he was apparently not the first person cast in the role. When he first heard that the film was being made, Hansen—then a graduate student in Austin—was told he’d be “great” for the role, but that it was already cast. Then the original Leatherface quit.

“Two weeks later,” Hansen recalled, “the same guy calls and says, ‘The guy who was hired as the killer is holed up drunk in a motel and won’t come out. There’s a lot of bad karma surrounding this movie, and I’m quitting.’ So I called [art director] Bob Burns and told him I was interested.”

Hansen—who stood six-foot-four and weighed 300 pounds—won the role from Hooper on sight.

6. LEATHERFACE WAS INSPIRED BY REAL MENTAL PATIENTS.

With no real dialogue (apart from a gibberish scene that Hooper eventually cut) to drive his character, and his facial expressions hidden by a mask, Hansen had to come up with other ways to express who he thought Leatherface was. When Hooper wanted the character to “squeal like a pig,” Hansen went out into the country and studied a friend’s pigs. Then, to capture the mental instability of the character, he went to an Austin mental hospital and studied the movements of the patients there, which he then incorporated into his performance.

7. TOBE HOOPER REALLY WANTED A PG RATING.

Despite its reputation for gruesome mutilation and gore, much of the violence in The Texas Chainsaw Massacre is suggested rather than directly depicted. This is because Hooper was hoping for a PG rating so that the film could reach a wider audience (there was no PG-13 at the time) and was told by the Motion Picture Association of America that he could help his cause if he limited the amount of onscreen blood.

“As you watch the film, notice there’s probably about two ounces,” Hooper later joked.

Alas, the film’s intensity ultimately meant it earned an R rating. Still, it’s probably not as gory as you remember.

8. THE NARRATOR IS A YOUNG JOHN LARROQUETTE.

The film’s menacing opening narration is an instant tone-setter, preparing the audience for a truly horrifying experience. The voice providing that menace? John Larroquette, then an unknown actor who was referred to Hooper by a friend. Hooper asked Larroquette to imitate Orson Welles for his reading, and while he didn’t quite get that, what the actor ultimately provided worked wonders.

9. THE SHOOT WAS HARROWING.

The Texas Chainsaw Massacre was produced on a budget of $60,000 raised by Bill Parsley, a Texas Tech administrator and former member of the Texas Legislature who fancied himself a film producer. Even in 1973 it was a shoestring budget (John Carpenter’s famously low-budget Halloweenwas made for five times that amount a few years later), which meant little pay and long hours for the cast and crew. To make matters worse, the production endured a Texas summer with temperatures in excess of 100 degrees (including 115-degree heat for the un-air conditioned interior shots), a single bathroom shared by more than three dozen people, costumes that could not be changed because the actors only had one set of clothes, and the constant presence of the bones and rotting meat used as props. Virtually no member of the cast went uninjured, and the heat and stench got so punishing at one point that the actors would run to the windows of the house where the dinner scene was shot to throw up and breathe a little fresh air between takes.

Years later, Hooper sarcastically referred to the experience as an “interesting summer."

10. THE LEGENDARY DINNER SCENE WAS SHOT IN A SINGLE MARATHON DAY.

The dinner scene near the end of the film in which Sally (Marilyn Burns) is terrorized by Leatherface and his family is one of the most intense sequences in all of horror cinema. It feels like you’re actually watching a group of people going insane, and that’s because … well, maybe you are.

In addition to the excessive heat and odor in the dining room during filming, the sequence was given another challenge: It had to be completed in a single day because John Dugan, the actor who played Grandpa, refused to endure the 10-hour process of getting his makeup applied a second time. “He announced that he was not sitting through it again,” Hooper said.

As a result, the cast and crew worked for 27 straight hours to finish a scene that takes up only a few minutes of the film’s runtime.

11. THE CAST ACTUALLY DISLIKED FRANKLIN.

For the role of Franklin, Sally’s wheelchair-bound brother who draws the ire of the audience when he grows angry with his more able-bodied friends simply because he can’t share in their fun, actor Paul Partain opted to take a very Method approach to his work.

“I was a young, inexperienced actor who didn’t realize that it wasn’t like theater," Partain latersaid. "You didn’t have to stay in character all the time. When I first read the part, I could see that nobody wanted this guy to be there. It just hit me that he was whiny.”

Partain’s commitment worked just as well behind the camera as it did in front of it. At one point he and Burns stopped speaking to each other between takes, and Hansen later recalled that Franklin was the only character he was actually happy to kill.

12. LEATHERFACE’S VICTIMS TREATED HIM AS AN OUTSIDER BEHIND THE SCENES.

As a large man who had to work every day in triple-digit heat while wearing a wool costume that he couldn’t change out of, Gunnar Hansen already had it rough while making The Texas Chainsaw Massacre. He got so smelly by the end of production that the rest of the cast and crew avoided eating around him. To make matters a little more difficult, though, he also dealt with an interesting character technique that his victims engaged in. During the shoot, Burns and the other kids who would eventually fall prey to Leatherface avoided Hansen because they didn’t want to hang out with their killer.

“During the filming, none of them would talk to me or be anywhere near me until they were dead,” he later recalled.