Rebel Wilson Says She Lost 8 Pounds During 4 Days of Filming on 'Cats' (Exclusive)

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Things were pretty steamy on the set of Cats!

The cast of the upcoming musical sat down with ET's Rachel Smith on Thursday, dishing on everything from their lessons at "cat school," to which cast members brought real cats on set to study!

For Rebel Wilson, who plays Jennyanydots in the Tom Hooper-directed epic -- based on the renowned Andrew Lloyd Webber musical -- it wasn't hard to get into the mischievous feline spirit.

"Apparently, I was deemed the naughtiest cat at cat school," she joked. But it wasn't all feline fun and games for Wilson and her co-stars, who put in some real-life sweat on their major dance sequences.

"I lost eight pounds, shooting my number, in four days," Wilson recalled. "One, because there's a lot of physicality... but also, they heated up the set very high, to almost 100 Fahrenheit, so that we could never cool down. These people are like, the best dancers in the whole world, so they can't cool their muscles down or they could get an injury and they'd be out of the film... So they'd heat up the set like a sauna so we would never cool down, but made it pretty uncomfortable."

The film caps off a banner year for the Australian actress, who starred in three other films -- Isn't It Romantic, and The Hustle, both of which she produced, as well as awards contender Jojo Rabbit--  and multiple TV projects, but for her co-star, Robbie Fairchild, who plays Munkustrap, the big-screen feature is a career first.

"For this to be my first film... being there every single day and watching Rebel, Judi Dench, Ian McKellan, Taylor Swift, Jennifer Hudson, James Corden, the list goes on and on," raved the former New York City Ballet principal dancer and 2015 Tony nominee. "And not only the stars, but every single ensemble member is some of the greatest talent in London and around the world."

"You definitely felt how epic it was," Wilson agreed. "From the sets and Tom Hooper directing... and then Andrew Lloyd Webber, who is a musical theater god."

"He would pop up on set and he'd just be hiding in the shadows, and all of a sudden you'd be singing and like, 'Oh, it's Andrew Lloyd Webber, geez better be in key!' The artistry that's gone into this film, and to know that it's all live singing and dancing. Spectacular is the right word to describe the film. It's epic."

Their "cat school" training was all about learning to move like a feline, but the Cats stars also took some clues from the real-life animals -- despite both Wilson and Fairchild being allergic! They watched YouTube videos to study movement and even palled around with an on-set feline friend, a hairless cat that was adopted and brought to set by dance duo Les Twins.

"It was actually really helpful, because it didn't have any fur, so you were able to see the way its muscles moved," Fairchild recalled.

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In between set-ups, the pair palled around -- making up choreography to perform in the background of other scenes, playing Bananagrams with Dench and navigating the complex "calibrations" of their CGI suits. Now, as buzz for the film continues to ramp up, they can't wait for fans to see the finished product.

"This is the first time they've used technology where it's your full CGI body on screen, and they're actually using your face from the footage of the film. That's why it's so revolutionary, because usually when you see a CGI character, they're full-computer," Fairchild explained. "Tom wanted to see emotion and reaction to something, very human that you don't get from a computer."

"It's so cool," Wilson agreed, "and I love seeing the videos on Instagram of kids making up their own dances -- even the style of the choreography of the film and everything was just cutting edge."

Cats is in theaters on Dec. 20. See more in the video below.

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