'The Substance' stars discuss that 'beautiful' bloody finale (spoilers!)

Spoiler alert! We're discussing important plot points and the ending of “The Substance” (in theaters now), so beware if you haven’t seen it.

Margaret Qualley can be quite the monster when she needs to be.

Demi Moore endured her share of makeup wizardry in the body horror movie “The Substance," playing an aging TV fitness celebrity who takes a black-market drug for an infusion of youth. But it’s her co-star Qualley, in the film’s bloody, bonkers finale, who wore a special prosthetic suit to play a creature grotesque in one way yet lovely in another.

“It was torture,” Qualley says. “I had this awesome team of prosthetic artists that put it on me and took it off of me and got me through the day and made me laugh a couple of times while I was just on the brink of panic.”

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In “The Substance,” Elisabeth Sparkle (Moore) gets pushed out of her job when her skeezy boss (Dennis Quaid) wants somebody younger and hotter. Elisabeth signs up for a multi-step DIY treatment process to become “more perfect,” the result is a younger new self (Qualley), and they have to share an existence as “one” – seven days for the old body, seven for the new, rinse and repeat.

Calling herself Sue, the other persona is cast as Elisabeth's replacement and becomes an instant hit for the network, so much so that she’s hired to host a New Year’s Eve live special. Sue likes the parties and the attractive dudes that come with fame, and she wants more than just a week. She begins to break the rules and take extra “Stabilizer” fluid from Elisabeth’s spine. This ticks off Elisabeth when parts of her body start withering away and growing exponentially older the more Sue messes with the status quo.

Sue is forced to let Elisabeth take over when she runs out of fluid, and Elisabeth orders a “Termination” serum to get rid of Sue. She gives her a shot and instantly has second thoughts, reviving Sue, but then she kills Elisabeth for her actions. Sue shows up to the New Year’s show but her teeth fall out and her body deteriorates, leading her to use the “Activator” drug that spawned Sue from Elisabeth out of sheer desperation.

Enter Monstro Elisasue, a B-movie monstrosity that’s a combo of the two women who, interestingly, seems to appreciate her body more than Elisabeth or Sue. Qualley adores one “beautiful and touching” moment where Monstro proudly puts in an earring on what looks like the front of her face. (One of the weirder aspects of Monstro’s body is Elisabeth’s silently screaming visage protruding from her shoulder area.)

“It was special to be able to finally feel love and gooeyness and have that come from a place of being like totally physically weird, though I find Monstro kind of gorgeous, too,” Qualley says. “It was complicated because it was internally very moving and externally, well, moving in its own way.”

To become Monstro, Qualley spent six hours in a makeup chair to put on the prosthetics, and her head couldn’t do much inside the suit. “I only have one eye. I can't hear anything. I can't move my arms. I've got these retainers in that are like too huge, they just kind of cut everything,” she explains. “It was grueling to embody. But the purity of the soul at that moment was so refreshing because I'd been playing (Sue) for four-and-a-half months by that point, who was really hard to relate to. Like really soulless, man.”

Moore adds that "it is an easier read on paper." Once the prosthetics are on, "you forgo eating. You can barely drink. You have to really be disciplined."

Monstro is also center stage in the movie’s most outrageous scene at the end, when the new hybrid creature goes onstage at the New Year’s show. The crowd freaks out, especially when they get covered in the blood that spews out wildly from Monstro’s exploding and regenerating body.

Qualley reports they used 30,000 gallons of fake blood, and writer/director Coralie Fargeat “actually had a firehose that she wanted to operate herself.” Qualley texted videos of the insanity to her co-star. “That was the only week I had off,” Moore says, laughing. “And I got shingles.”

Monstro doesn’t exactly get a happy ending – she escapes but definitively blows up all over Elisabeth’s star on the Hollywood Walk of Fame in the film’s final scene. But she did stick with Qualley afterward: Wearing the Monstro suit caused acne to break out on Qualley’s face, which you can see in the next role she filmed in “Kinds of Kindness.”

“I hadn't recovered. That was six months later and it was still just like, yeah, Monstro lives on,” Qualley says. “But honestly, I kind of loved the way that looked so it was OK.”

This article originally appeared on USA TODAY: 'The Substance': Demi Moore, Margaret Qualley unpack that ending