“Terrifier 3” has surpassed all expectations at the box office. But behind the scenes, only a handful of people are responsible for the gory sequences that make Stephen King look kid-friendly.

The Post spoke to Christien Tinsley, the Oscar-winning makeup and prosthetics specialist who worked on the slasher movie, and special effects supervisor Jason Baker, who shared all the details about how they helped director Damien Leone’s graphic and bloody scenes come to life.

They also addressed the horrific final shower scene, which Baker called “pretty, pretty nasty,” that left many moviegoers vomiting or exiting the theater at the UK premiere. Tinsley revealed it was the last scene they shot — and the fake blood was running out.

For those who haven’t seen the flick, Art the Clown bombards two nude lovers with a chainsaw as they shower, cutting each limb by limb — and it’s one of those scenes that will make you want to chuck your lunch.

“One of the things [Leone] told me early on that he hadn’t fleshed out in the script … was ‘shower scene kill.’ And that’s about all he had in there. I was like, ‘What happens?’ And he was like, ‘I’m still trying to figure it out … but this has to be the big kill,'” Tinsley told The Post, adding, “I know we had to level up. We had to give something that the audience has never seen.”

Regarding how much fake blood they used in the movie, Tinsley said, “If I had to take a wild guess, we probably went through a couple of drums easily on that film,” where each drum contains 55 gallons. He admitted the shower scene alone was about “20 gallons of blood.”

They used so much blood on the film before shooting the “big kill” scene that Tinsley and his team “wiped out every grocery store in a 10-mile radius,” Baker revealed.

Tinsley confirmed that he and another crew member had to run around town for materials in the eleventh hour to make more blood.

“We got to that very, very last day — we were so close and we realized we were going to run out of blood. So me and my partner on set, Ryan Ward, we went to Walmart, we went to the grocery stores, we went to everywhere, and we just bought all the Karo syrup we could get,” Tinsley said, revealing they mixed the syrup with pigments from his Los Angeles Tinsley Studio that he got shipped last-minute to the movie location.

Ultimately, they had just enough for Art the Clown to make his demonic blood angel.

“The very last scene that we did in the movie was Art doing the snow angel in the blood,” Tinsley said. “We still had a ton of blood left over in the shower, just still on the floor. And so then it became a conversation of, ‘Nobody walk here. We want to keep as much of this blood on the floor as possible.'”

Tinsley said they took what little blood they had left and “very judiciously” poured it on the floor to “give us enough of a blood pool that [Art the Clown] could lay down and do his blood angel.”

Tinsley also revealed that the crew was “covered in blood” at the end of every day.

“Everybody’s walking out of that room rinsing off their shoes,” he laughed. “Damien just threw away his shoes at the end of the night.”

Baker said, “I keep telling everybody, I’ve been doing this for almost 20 years and ‘Terrifier 3’ was the first movie where I couldn’t get the blood out of my shoes at the end of the day and just threw them away.”

Leone’s “Terrifier 3” is breaking records everywhere. The low-budget slasher beat Lady Gaga and Joaquin Phoenix’s “Joker: Folie à Deux” for the No. 1 slot in North America, topping the box office at $18.3 million during its opening weekend earlier this month.

While the horror-franchise director told Variety that he “can’t see” the “Terrifier” saga “going further than one or two more films,” Tinsley isn’t too sure.

“Going into this one, [Leone] goes, ‘I got one more left in me,'” Tinsley told The Post. “You don’t want to hold anybody to that.”

“He’s planning on ending it on the fourth,” he continued, but “you know how these things go. You get enough success and enough money behind something, there’s always a way to keep it going.”