Andy Muschietti's IT, based on Stephen King's novel, has been scaring huge audiences for weeks now, and much of the credit for those scares must surely go to the practical and digital effects teams.

While actor Bill Skarsgård underwent a lengthy prosthetics and make-up effects effort to become Pennywise, many of the most horrific scenes that could not be achieved with practical effects alone were completed via incredible digital artistry.

SYFY WIRE reached out to one of the lead visual effects studios on the film, Rodeo FX, to learn how some of the key CG shots were got made. That includes Pennywise's kill mouth, the creature's mind-boggling shape-shifts — like out of the refrigerator — and the film's dramatic final fight.

A frightening fridge monster

At one point, Pennywise emerges — fully contorted — from a refrigerator. "The idea of a character being broken and packed into his face and then unfolding -- I thought that was really interesting," observes Rodeo FX visual effects supervisor Arnaud Brisebois. "I knew it was going to become an iconic horror shot."

To make the terrifying shot a reality, Skarsgård was filmed lying on the bottom of the fridge on his back. The actor was attached to wires that pulled him out of the fridge and rotated his body as he stepped out.

Rodeo FX's team then built CGI portions character that allowed them to move limbs and body parts in any way, hand-animating the unfolding. Only Skarsgård's real face was preserved.

"I actually suggested to my animation supervisor Yvon Jardel that he animate in reverse," says Brisebois. "So Pennywise would step back into the fridge so that he can push himself inside. It gave a better choreography to enter and fold onto itself than to try and do it the other way."

Crafting a kill mouth

Credit: Warner Bros.

Pennywise brings out his shark-like gums and rows of teeth more than a few times in the movie. To make this possible, the visual effects artists at Rodeo FX devised a ribbon-shape "rig" that multiplied the number of teeth inside the creature's mouth.

"The teeth are embedded in the gum, but they're hidden in scale," explains Brisebois, who worked with overall visual effects supervisor Nicholas Brooks on the film. "The teeth rig was all hand-animated. We basically modeled the teeth all individually or as clusters that could be grown and oriented in any way."

Rodeo FX also added in face and skin rippling, which had been part of previously created concept art. "It was something that Andy really liked, kind of a foreskin quality to his face as it pulls back," says Brisebois.

"There are even a few shots with the kill mouth where Bill Skarsgård's face is entirely replaced," adds Brisebois. "In some of the early frames we kept his actual eyes just to create a small connection."

Picture perfect

A painting of a woman that comes to life also involved digital intervention from Rodeo FX. The woman in the painting was played by actress Tatum Lee, and the look inspired by Italian-Jewish artist Amedeo Modigliani (who had also inspired Muschietti's titular character in his first horror film, Mama).

The woman, known as Judith in the film, was originally only going to involve some minor visual effects augmentation, essentially to warp her eyes further apart. Rodeo kept adding to the character to the point that most of the performance was somewhat replaced. In the final fight scene, Judith was also given a kill mouth.

Video of IT - Pennywise Featurette

Brisebois says he was particularly happy with how Judith turned out, especially as the character was nearly cut from the film (she does not appear in the book). "Andy fought for it because it was a bizarre character. It wasn't really scary -- it was more about discomfort. We would keep sculpting the character and iterating on very minute details of her facial features right until the end."

Tower of children

Deep in the sewers where Pennywise hides, a tower of toys and floating kids began as a live-action set 16 feet high that was digitally extended by Rodeo. The children were CG and the visual effects studio went all-out to make sure each looked different.

"We could randomize texture variations and weathering of the clothing for the children," notes Brisebois. "We had about 7,000 possibilities of variations for what ended up being about 75 or 80 kids. We even simulated hair for them."

A showdown with Pennywise

Having terrorized the children of Derry for some time, Pennywise ultimately finds himself under attack in the sewer when the kids fight back. During the scene, a number of weapons are used, and Pennywise also transforms into several beastly incarnations.

The fight would be complicated to shoot, and so an effort was made to plan out the choreography in a kind of previsualization, or previs. "Basically what they did," explains Brisebois, "was bring in all the stunt performers into a gym, and they learned the choreography and they motion-captured this. We built really quick CG assets for all the characters, including Pennywise, and we planned out all his transformations and the action."

Credit: Warner Bros.

When Pennywise did have to shape-shift, or grow creature arms, or bend and twist in unimaginable ways, Rodeo FX would augment Skarsgård's performance with 3D creature pieces, morph different creatures together, or take it over completely digitally. At one point he even becomes the Modigliani woman.

"In the end," says Brisebois, "you could have the shape of both characters, but both of them could have the textures of one or the other. With some simulated effects we'd be able to balance all of that out and really time it to what Andy was looking for."

There are a lot more digital effects in IT — by Rodeo FX and other visual effects studios — but Muschietti certainly found a way for them to be seamlessly incorporated, along with practical effects, in order to conjure up more scares.

"It's great to see so many people enjoying the film and watching everything we had invested so much effort in," says Brisebois.