The actress was at a Cannes Film Festival press conference this weekend after the premiere of "The Substance," the new flick she's in, and Demi said stripping down for the role required loads of vulnerability and rawness ... something she felt she needed to do to tell the story.

She gives a shout-out to her costar, Margaret Qualley, whom she says eased her nerves and served as an incredible scene partner for the moments they had to get fully naked.

In one particularly graphic scene ... DM engages in a naked, no-holds-barred fight with Qualley - which Demi says ended up being light-hearted fun BTS thanks to their camaraderie despite the vulnerability attached to the scene.

At the end of the day, Demi says the film totally pushed her outta her comfort zone ... but she knew the explicit scenes were necessary for the film -- and luckily, director Coralie Fargeat approached it all with sensitivity ... establishing a common ground of mutual trust.

"The Substance" is said to tackle beauty standards, toxic masculinity, and female self-hatred ... with Demi stepping back into the spotlight as Elisabeth Sparkle, a fading youth-obsessed celeb who turns to a black market drug.

The potion conjures up a younger, upgraded version of herself, played by Margaret -- and it sounds like things get spicy between her and Demi on camera.