Moore stars as Elisabeth, an ageing TV fitness instructor a la Jane Fonda who signs up for a mysterious medical regime called The Substance that promises to create the perfect version of herself - played by Margaret Qualley of Drive-Away Dolls.
The role requires Moore, 61, to be completely naked as well as transform into a deteriorating old woman through prosthetics.
"All of it at different times had moments that were challenging," she told Reuters in an interview.
"The level of vulnerability that this role called for on all the different levels - emotionally, physically - were as demanding as it was exciting because it was really pushing me to step out of my comfort zone," Moore said.
Critics were positive on Moore's performance, with entertainment website Variety calling it "nothing short of fearless" and The Hollywood Reporter praising how she "imbues her character with a visceral desperation."
This year's festival marks the first time in more than a quarter-century that Moore, who became a sex symbol through films like Striptease in the 1990s, was at the festival.
For Qualley, 29, having to portray the so-called perfect version of a woman's body was a weird experience.
"She is meant to be perfect, but she's probably like the least beautiful character I've ever played because she's heartless," she said.
French director and writer Coralie Fargeat said her goal was to explore women's toxic relationships with their bodies and how they are taught that their worth is tied to their appearance.
"The way I wrote the movie really sticks to what we experience as women with our bodies. So, it's the way our body is seen, but also the way we see our own body," she said.
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