McKellen opened up to ET about the upcoming musical adaptation at the press day for his new movie, 'The Good Liar.'
For Ian McKellen -- who plays Gus the Theatre Cat in the upcoming musical -- this uncanny combination of human performer and CGI cat is actually important to the point of the project, and is a perfect stage-to-screen translation of having actors wearing cat costumes.
ET's Rachel Smith recently sat down with McKellen -- alongside his The Good Liar co-star, Helen Mirren -- at the New York City press day for their upcoming mystery drama, where he opened up about his upcoming work in Cats and the film's polarizing graphics.
"The stage show, Cats, was not about a lot of people being convincing as cats, but it was about human beings discovering their cat-like nature," McKellen shared. "And it was hugely successful."
"When it was now being done in film, the temptation I think must have been to turn those actors into cats, like Lion King, and make them look like real cats, but they're not real cats," he added. "They're people playing cats, and that is the notion of the film, and it's been done very wittingly, I think, and particularly the dancers."
"To see a young person discovering the cat in them, your jaw just drops with delight," McKellen added. "Dame Judy [Dench] and I and James Corden and all the rest, Taylor Swift, we're all busy doing our version of cats."
The 80-year-old movie star quipped that not only does he perform "high kicks galore" in the upcoming film, he also signed on to some future follow-up projects.
"I'm going to make the sequel, which is called Dogs! Then we're doing Ferrets, I think," McKellen joked. "[Then] Hamsters... we're doing a wonderful show called Hamsters."
Meanwhile, ET recently spoke with McKellen's Cats co-star, Rebel Wilson -- who plays Jennyanydots in the upcoming musical -- at the premiere of her new comedy, Jojo Rabbit, in October, and Wilson opened up on how she felt about the pushback against Cats' use of groundbreaking CGI.
"It's a new technology. When people saw Avatar they were also like, 'What is that?' Wilson shared. "So I think there was a little bit of that going on when they saw the Cats trailer because it's such a brand new technology."
"But personally I've seen bits of the film it's incredible it's amazing," Wilson added. "I can't wait for you guys to see that when it comes out in Christmas."
However, before Cats -- or any of McKellen's fictitious-but-awesome ideas for potential sequels -- hits theaters on Dec. 20, he and Mirren will be sharing the big screen in The Good Liar, which also marks the first time the pair have appeared together in a film.
In The Good Liar, McKellen stars as a career con artist who targets Mirren, a recently widowed woman now worth millions, by posing as a suitor through an online dating site. However, things get complicated as the two grow closer and their secrets are threatened.
So, why did it take so long for the two beloved Hollywood luminaries to come together? According to Mirren, "We never got asked."
"We would have done it years ago if someone had asked us," Mirren added.
"But you know, we're not wholly in charge of our own career," McKellen added. "You have to wait for the phone to ring!"
McKellen added that, having not seen the pair on screen together before "does add a piquance to the film" because of its novelty.
"Absolutely," Mirren said. "If I'd been Mrs. Gandalf or something, [people would think], 'Oh, here they come again.'"
The Good Liar -- directed by McKellen's frequent collaborator and Oscar winner, Bill Condon -- hits theaters Nov. 15.