Due to time constraints, Persin subcontracted the build of Monstro Eliasue’s suit to a U.K.-based company. “We had two full suits, one for a stunt double, and a half-built suit for Margaret, which she wore for closeups.” Persin and his team took care of all the effects, including the boob that squeezes out of one of Monstro’s many holes.

Demi Moore tells PEOPLE she hadn't met Margaret Qualley before becoming her costar in Coralie Fargeat’s new horror movie 'The Substance,' but had enough 'indirect connections' that they 'felt like we

Despite the film's darker themes and gory visuals, Moore and Qualley created an environment where they could find comfort and be their best.

Obviously, between all of the scenes of spine-splitting, vomiting, close-up injections and Dennis Quaid chowing down on a plate full of prawns, the whole film wasn’t exactly a walk in the park, but there was really no preparing for the birth of Monstro Elisasue in the final act, was there?

Demi Moore, Margaret Qualley, Dennis Quaid and more stars hit the red carpet for the L.A. premiere of their film, 'The Substance.' See all the photos.

From daunting high-energy choreography to nine-hour prosthetic application, the stars tell EW "The Substance" was a satisfyingly grueling, months-long shoot.