In the late 1970s, Wes Craven was a struggling filmmaker known for only one thing: a little horror flick called The Last House on the Left (1972). Though he was itching to branch out and make other kinds of movies, he could only find financing for horror films, so he agreed to make a movie about a group of hill people savaging a vacationing family. Though he may not have been in a hurry to admit it, Craven found that he was really good at scaring people.

Produced on a tight budget, under sometimes grueling conditions, The Hills Have Eyes cemented Craven as one of Hollywood’s great horror masters. The film was released 40 years ago today, and it’s just as brutal as ever. So let’s look back on its unflinching terror with 11 facts about the film’s production.

1. IT WAS BASED ON A TRUE STORY.

According to writer/director Wes Craven, The Hills Have Eyes was inspired by the story of Sawney Bean, the head of a wild Scottish clan who murdered and cannibalized numerous people during the Middle Ages. Craven heard the story of the Bean clan, and noted that the road near where they lived was believed to be haunted because people kept disappearing while traveling on it. He adapted the story to instead be about a group of wild people in the American West, and The Hills Have Eyes was born.

2. IT WAS INSPIRED BY NECESSITY.

After Craven released The Last House on the Left in 1972, he tried his hand at making films outside of the horror genre, but according to the late director, “Nobody wanted to know about it.” In need of money and searching for a better career path, he finally answered the request of his friend, producer Peter Locke, to write a horror film. At the time, Locke’s wife Liz Torres was performing regularly in Las Vegas, and so Locke was frequently exposed to desert landscapes. He suggested that Craven set the film in the desert, and Craven began to craft the screenplay.

Budget was also a concern, so Craven structured the film to feature a relatively small cast and very few locations.

3. JANUS BLYTHE WON HER ROLE BASED PARTLY ON SPEED.

For the role of Ruby, the filmmakers needed an actress who could pull off the flighty and feral character convincingly, so, in the words of Locke: “We had sprints.” Actresses trying out for the role were asked to race each other, and Blythe’s speed won out.

4. PETER LOCKE PLAYS A SMALL ROLE IN THE FILM.

Because of the film’s small budget, even Locke was drafted to join the cast. He appears as “Mercury,” the feather-covered savage who appears only twice: once in the film’s opening minutes, and then again as he’s pushed off a cliff by the Carter family’s dog, Beast.

5. THE TARANTULA SCENE WASN’T PLANNED.

The scene in which Lynne Wood (Dee Wallace) discovers a tarantula in the family trailer is a foreboding moment that signals the trauma to come, but it wasn’t in the script. According to Craven, they simply found the spider on the road during shooting, put it in a terrarium, and decided to add it into the film. Don’t worry, though: Wallace didn’t actually stomp the spider in the scene.

6. THE DEAD DOG WAS REAL (BUT THEY DIDN’T KILL IT).

During the scene in which Doug (Martin Speer) discovers the mutilated body of the family’s other German Shepherd, Beauty, a real dog corpse was used. According to Craven, though, the dog was already dead.

“Let’s just say we bought a dead dog from the county and leave it at that,” Craven said.

7. THE FILM WAS ORIGINALLY RATED X.

Though it might seem relatively tame by modern standards, the film’s graphic violence earned it an X (what we now call NC-17) rating from the MPAA, which meant cuts had to be made. According to Locke, significant footage was removed from the scene in which Papa Jupiter (James Whitworth) kills Fred (John Steadman), the scene in which Pluto (Michael Berryman) and Mars (Lance Gordon) terrorize the trailer, and the final confrontation with Papa Jupiter.

8. MICHAEL BERRYMAN CONSTANTLY FACED HEATSTROKE.

Berryman, who became a horror icon thanks to this film, was apparently game for just about anything Craven and company wanted him to do, though he personally told the producers he was born with “26 birth defects.” Among those birth defects was a lack of sweat glands, which meant that the intense desert heat was particularly hazardous to his health. He soldiered on, though, even in intense action sequences.

“We always had to cover him up as soon as we finished these scenes,” Craven recalled.

9. THE CLIMACTIC EXPLOSION COULD’VE BEEN DEADLY.

Because the budget was small, production on The Hills Have Eyes often meant taking risks. Actors performed stunts themselves, sometimes putting themselves in harm’s way. For the scene in which Brenda (Susan Lanier) and Bobby (Robert Houston) set a trap to kill Papa Jupiter by blowing up the trailer, the crew members who set the explosion actually couldn’t tell Craven whether it was safe to have the actors in the foreground of the shot.

“We didn’t know how much of a blow-up it was gonna be,” Craven said.

10. THE ORIGINAL ENDING WAS MUCH MORE HOPEFUL.

According to Locke, the film’s original scripted ending involved the surviving family members reuniting at the site of the trailer, including Doug and the baby, signifying that they had survived and could finally look forward. Craven, though, opted for something more bleak, and so the film ends on a shot of Doug brutally stabbing Mars while Ruby looks on in disgust, a reversal of roles that the director liked.

11. IT STARTED AN INTERESTING CHAIN OF HORROR HOMAGES.

The Hills Have Eyes is admired by fellow horror filmmakers, so much so that one of them—Evil Dead director Sam Raimi—chose to pay homage to it in a strange way. In the scene in which Brenda is quivering in bed after having been brutalized by Pluto and Mars, a ripped poster for Steven Spielberg’s Jaws is visible above her head. Raimi saw it as a message.

“I took it to mean that Wes Craven … was saying ‘Jaws was just pop horror. What I have here is real horror.’”

As a joking response to the scene, Raimi put a ripped poster for The Hills Have Eyes in his now-classic film The Evil Dead (1981). Not to be outdone, Craven responded by including a clip from The Evil Dead in his classic A Nightmare on Elm Street (1984).

Additional Sources: The Hills Have Eyes DVD commentary by Wes Craven and Peter Locke (2003)