Who would ever want to turn Demi Moore into a monster? That's exactly what director and writer Coralie Fargeat does in Moore's latest film. The substancewhich is now in cinemas. And indeed, this transformation is a career-defining role for Moore.

In the film, Moore plays Elisabeth Sparkle, an actress who is fired from her long-running fitness show on her 50th birthday. In her grief over losing her job and growing older, she begins taking a drug called “The Substance” that promises to transform her into a better and younger version of herself.

Taking The Substance is not quite what it seems. Essentially, Elisabeth gives birth to a younger version of herself, named Sue and played by Margaret Qualley, in a rather grotesque manner. There are processes that must be followed to keep Sue and Elisabeth apart. When Sue goes berserk and ignores these rules, unexpected, monstrous side effects occur, slowly changing Elisabeth's body in disgusting ways.

At various points in the film, prosthetics are used to transform Moore's body into a hunchbacked man with no hair. Her kneecap looks like a chicken leg with bright blue veins protruding from it at other points. An open wound at the base of her spine slowly becomes infected over the course of the film and oozes green pus.

Special effects artist Pierre Olivier Persin is the person who drives Moore’s physical development throughout the film. (His previous work includes game of Thrones And Avengers: Infinity War.) He tells The Daily Beast's Obsessed that 70-80 percent of Elisabeth's physical changes are prosthetics and the rest are visual effects — which was something Fargeat wanted. “What I liked most was that she wanted to use as many practical effects as possible,” he says. “I love visual effects, but in a world of AI, this was really a big change. Sometimes I would push to use visual effects and CGI, and she would just say, 'No, no, no.'”

They worked together for nearly a year on the prosthetics for the film. Persin was also on set and says the filming was just as intense as writing the film's script and developing Moore's Elisabeth.

“(Moore) was a real fighter,” he says. “She never complained. She was with us the whole time, looking in the mirror and observing every detail while sitting quietly. (She was just) the dream actress for us.”

Depending on which version of Elisabeth was being filmed, Moore had to wait every day for the prosthetics to be fitted. This took between 45 minutes and almost six and a half hours. This was necessary for a shower scene in which Elisabeth reacts to the new version of her body, which consists of a flabby liver, blotchy skin, yellowish thick nails and a bulging back. “She has a complete leg, arm and back prosthetic. Then the face and the wig. She also has to go under water and everything has to be waterproof,” Persin recalls of the scene.

As Elisabeth's body literally begins to deteriorate into a monster, the various suits they were working on were given appropriate nicknames.

“The first stages of Demi’s aging process were requiem for Requiem for the dream,” he says of scenes where Elisabeth's side of her body becomes too patchy and begins to age, a reference to Ellen Burstyn's character. The next phase, in which she gets a huge hump and saggy ass, was called Gollum. And the ultimate monster – the tit-filled hybrid of Elisabeth and Qualley's Sue – was called Monstro.

These transformations all begin with just one finger. After Sue disregards the specific rules for taking the drug, Elisabeth is left with a gnarled finger that is strongly reminiscent of the evil witch in Snow White. It was important to get the finger just right as it is only the beginning of Elisabeth's transformation for the rest of the film.

“We were doing stuff in the workshop and I decided to test the finger because that's the starting point of everything,” says Persin. “When I tested it, I thought it looked too big and silly and I hated it. We were building all the other prosthetics at the time and I interrupted everyone in the workshop saying, 'If the finger doesn't work, everything else that follows won't work.'” So they started remaking everything based on the finger; Persin says they probably remade everything about twice.

Persin and Moore also worked closely with the makeup and hair departments. After the finger, for example, one side of Elisabeth's body becomes wrinkled and her silky long hair becomes a wiry, frizzy, gray mess. During this phase, they designed sculptures for the one side of her body that will be deformed, while also working the hair into it. Persin then worked closely with the wig makers.

After Sue has neglected Elisabeth for what feels like weeks, the path to the monster is almost complete and Elisabeth turns out to be a witch-like creature – the evil witch in Snow White remains a good comparison, although Elisabeth now has a lot of wrinkled, flabby flesh, sagging breasts and a huge hump.

There was a double for Gollum, but Moore himself also wore the suit. Persin says the practical prosthetics were important to make the changing flesh more vivid. “I didn't want just a rubber suit, so it was silicone. It was more than a suit. It was more than prosthetics. Most of it was glued to the body so it could move well. The butt, for example, sagged a little. We had hollows in the breasts and the butt, so the butt would wiggle when you walked.”

But the Gollum is just a warm-up for the film's shocking final act. As Sue comes to terms with the changes to her own body, the drug transforms her into something entirely new – a gigantic monster of flesh, teeth and hair – that has Sue's one black eye and Elizabeth's frozen, screaming face.

Persin and his team took almost a month to design the hybrid of the two characters, affectionately called Monstro Elisasue. “The head is a bit like a female elephant man,” he says. “That's what Coralie wanted, the sensitivity of The Elephant Manthe David Lynch movie. We spent a lot of time designing Monstro with all the boobs and balancing everything. Is she fat enough? How many boobs? Maybe we should add a jaw here. Maybe we can (add a backbone), because there are a lot of backbones in the story from the beginning.”

They scanned the design and sent it to a British company that built the Monstro Eliasue suit. Persin and his team then constructed the Elisabeth blob that ends the film: the exploding heads and everything else that comes out of Monstro Eliasue's body in the bloody finale.

So much happens in this finale – Monstro presents herself to an audience in her best attire and then all hell breaks loose. It starts with Monstro, who, in a mixture of vomiting and giving birth, releases a single breast from inside her body. This breast is controlled by Persin, who uses puppets to release it from the suit.

“I love puppets. Technology is great, but sometimes you can create a very beautiful, organic feeling with puppetry. The chest that comes out of Monstro was something I operated as a puppet. After 10 attempts, I had no strength left in my arm because everything was quite heavy.”

Pulling a breast out of a monster's body isn't the wildest thing in the film's metal-music-accompanied finale. Eliasue appears on the New Year's Eve show that Sue was supposed to host, and she seems comfortable in her monster skin. But when the audience starts booing her, she freaks out – Carrie– Carrie White. There is blood and brutality everywhere, which was done by the special effects team. They built a contraption that shot a fire hose filled with blood into the audience, which was made up entirely of stunt people due to the power of the hose.

There was a full monstro suit for a stunt double, but since it was made of foam latex, it was soaked in blood and quickly turned pink. There was also a half suit that Qualley actually wore in the final scene to play Eliasue so her real eye could represent her only eye left after all the carnage.

“Coralie really wanted a performance, so all the close-ups were of Margaret,” says Persin. “She was a real fighter. During the prep, I was against it. I thought, 'It's just one eye and it's going to be very hard on her.' Coralie said, 'It's a performance. I want the actress.' She was right, because what's happening is really moving and strange, and we really need Margaret in the suit. So that was all Margaret.”

Fargeat also spent her own time in the Monstro suit. The shots of the blood splattering into the audience from Monstro's point of view show her holding the camera, headless but in the suit because the arm was in the blood gear.

The idea of ​​performance is very important to the film. When it ended with a blob with Moore's face on it, Fargeat wanted that expression. Persin and his team built a puppet blob that they moved for the scene, and with the help of visual effects, Moore's face was placed on it.

Praising someone for how disgusting and insane their work on a film is could be taken as an insult, but for Persin and his team, it's high praise. “I'm glad you say that, because we worked a lot on it.”