Making the absolutely bonkers body-horror film “The Substance” was an emotionally taxing, physically exhausting effort for Demi Moore and Margaret Qualley. Some days were definitely better than others, however.

Lying in a clump on the floor with your co-star in a state of undress might seem like one of those uncomfortable situations, but Qualley was all good with it. “I don't mind Demi Moore naked on me. Like, it's just not a problem. There's so much worse in the film,” she says, cracking her fellow actress up over a Zoom conversation.

“You probably felt more safe in those moments than some of the others,” Moore figures.

“I was fine with that,” Qualley adds. “Yeah, you want me to fall on Demi Moore naked? Sure. OK.”

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Writer/director Coralie Fargeat’s “The Substance” (in theaters Friday) has become the buzziest horror film of the year after garnering a 13-minute standing ovation at Cannes Film Festival and winning the Midnight Madness People’s Choice award at the recent Toronto International Film Festival. Moore’s go-for broke performance has also put her in the best actress Oscar conversation.

In the film, she plays Elisabeth Sparkle, a middle-aged TV fitness celebrity who's shoved out the door by her male boss (Dennis Quaid) in favor of someone younger. Elisabeth signs up for a black-market drug that promises to bring out her "younger, more beautiful, more perfect" version. A shockingly brutal process births Elisabeth’s younger self (Qualley), but they have to follow certain rules, most importantly switching bodies every seven days.

Calling herself Sue, the younger version is cast as Elisabeth’s replacement and becomes an instant star, and when she begins to bend those rules, a battle between the two leads to a wild and gore-filled climax.

Qualley, 29, reports that it’s been “cool to be in the room” with people wowed by Moore’s “unexpected” performance. “That's how I feel about Demi's career,” she says. “She goes from ‘G.I. Jane’ to ‘Striptease.’ It's all over the place and every single one, she's going into some uncharted territory, completely vulnerable, completely badass (and) utterly singular.”

Moore, 61, finds it “a joy and a pleasure" to have a script come her way with "such rich, complex characters” as well as “something that really pushed us out of our respective comfort zones.”

Adds Qualley: “I mean, understatement of a lifetime.”

During the grueling 5 ½-month shoot, both actresses underwent a bunch of prosthetics work – Elisabeth’s body gradually turns into more of a horror show due to Sue’s actions, and Moore would undergo upward of six hours of applications. But she also appreciated having her emotional vulnerability tested in nude scenes where Elisabeth studies her body.

“Part of what was attractive is I knew that this wasn't about highlighting my (body) and being lit in my most attractive ways, that in fact, it would be amplifying those things that you might see as flaws or that you don't want somebody to see,” she says.

Qualley’s own personal horror show was a sexy dance class that Sue leads, twerking and gyrating in a skin-baring pink leotard.

“Being a woman and having your relationship to your body is so specific, based on the cards that you're dealt,” Qualley says. In her career, “I’m super-lucky that I didn't have to use my body in that way because it doesn't come very naturally to me. That said, doing that dance, that was the most emotionally vulnerable, scary thing for me because it's outside of my comfort zone. It's not how I feel hot, but I had to ‘feel hot’ while doing it.”

And while their characters don’t share that much screen time, Moore and Qualley were usually working on the same days and acted as each other’s support system. “Demi would tap in, I would tap out,” Qualley says. “We would just pass each other and look at each other like, ‘Mm-hmm. You got this.’ ”

“The Substance” explores themes of age and sexism but it’s not just limited to celebrities: Moore sees a universal appeal in how we all view our bodies, men and women alike. The movie's Hollywood setting is the “perfect” way to communicate that, “where outside criticism is already heightened, that you're already on display," she says. “We've all stepped through that door of self-judgment, self-criticism, that kind of battle, and in many cases can lead to almost a violence against ourselves."

Qualley didn’t see herself in Sue very much – “I’m not like a soulless sexbot or something” – but agrees that everyone can relate to Demi's portrayal of Elisabeth. “The way this is portrayed is a satire, it's exaggerated, and yet it's effective because it's based on some level of reality,” she says.

There's "almost like an unspoken collective agreement about women's value diminishing as they get older, their place," Moore concludes. "But I don't like to view anyone as a victim. Certainly not myself.

“The hope is that maybe we can be part of laying down some new stones on that pathway to a cultural shift by creating just an awareness,” she adds. “But that awareness starts within each of us as individuals. We can't wait for the outside to catch up to us. When we change what we believe about our value, then thereby the outside world will reflect it.”