Dame Judi Dench and Sir Ian McKellen are among the all-star cast of the big-screen version of the musical Cats. They sit down with Laura Harding to talk about the seemingly impossible task of filming an un-filmable musical.

For decades, the musical Cats was thought to be un-filmable.

A smash hit in the West End and on Broadway, Lord Andrew Lloyd Webber's show based on poems by TS Eliot has become one of the most famous of all time, but never been made into a movie.

But that all changes with director Tom Hooper's interpretation, which uses the latest technology to turn stars such as Dame Judi Dench, Sir Ian McKellen, Idris Elba, Jason Derulo and Taylor Swift into furry felines.

"It's not only un-filmable, it's un-stageable," says Sir Ian with a chuckle.

"It's a ridiculous enterprise, but one of the most successful musicals in the history of world theatre - and I suspect it's going to be a classic film.

"I was overwhelmed with joy when I saw it, and I'm not really a cat person, I prefer dogs."

Sir Ian, who plays Gus the Theatre Cat, is leaning back in his seat, a dashing scarf draped round his neck, as he swaps gossip with his old friend Dame Judi.

"But there's so much talent on the screen," he adds, "and Judi and I were just sat down, for many days, watching the young dancers.

"They worked very long hours and never complained and were always good, whether they were tap dancing or doing classical or hip hop."

Indeed, the film blends a wide range of dance styles to accompany some of the classic songs of musical theatre, including Memory, Macavity and Mr Mistoffelees, and stars Royal Ballet principal Francesca Hayward and street dancers Laurent and Larry Nicolas Bourgeois aka Les Twins.

"Because I'm a devotee of the art form of pantomime," Sir Ian adds, "it's not an insult to say that Cats on stage is a bit of panto, which is a plus for me, but it's of that genre.

"Tom Hooper has managed to make it into something quite different and, for me, very, very engaging indeed because it's not only very funny at times; it's suddenly unexpected alluring, and not just because of the music, because of the performance and this wonderful dancing."

For his co-star Dame Judi, the chance to play Old Deuteronomy fulfilled a dream she first had in 1981, when she was due to take part in the first West End staging, but had to pull out due to injury.

"I thought it was the end," she says. "Because I was in plaster and I was playing Grizabella, they thought, 'It doesn't matter, she's a clapped out old cat', but I had fallen off the stage, so I went home and got my husband Michael to ring Trevor (Nunn, the director) and say, 'I can't do it'.

"So, I never thought it would come back and it has. I'm thrilled!"

While taking on the part on stage would have meant heavy stage make-up and a big furry costume, the film was a different proposition altogether.

Sets were built extra large to make the actors appear small and it uses CGI and "digital fur technology" to create a feline body, whiskers, ears and a tail for each of the characters, meaning that the actors dressed in green leotards so their fur could be added in post-production.

"It was all a new experience for me," Dame Judi says. "With a play or something you're about to do, you know what that person will look like.

"And in this case, of course, we didn't quite know."

"We didn't have tails, we didn't have ears, we didn't have whiskers..." Sir Ian points out.

"We didn't!" Dame Judi adds. "We just had a lot of green leotards and spots.

"You look at things and think, 'How are they going to get rid of that?' But of course they can do wonderful things.

"I don't know how many people work on it afterwards, but it would very nice to do this in the theatre - get rid of anything that they don't want.

"I thought I knew what I looked like and quite suddenly I looked like a glorious orange show cat, which I am very, very pleased about."

In fact, there was something rather familiar about her appearance.

"I look very like a cat I had once called Carpet," she says, "so it's in memory of Carpet."

While the appearance of the cats has been the focus of much of the attention since the film's first trailer debuted over the summer, Sir Ian was bowled over by something else entirely.

"Grounding everything is the performances, which work without the make-up, without the fur, so with the fur ... it's even better!"

Also marvelling at the scale of the film is pop star Derulo, who plays Rum Tum Tugger.

"I remember my first time walking on set," he says excitedly," and I was like, 'This is crazy!' because I've never seen a set like that in my life.

"The scale is so large and 90% of the set is all built out, it's not CGI, and I was blown away at new technologies.

"This one is just so different, it's so left field that it causes a pulverising response from people and I think it's incredible because the stage play did the same thing.

"It was a very pulverising musical, so when people saw it from the outside looking in they were like, 'What is this?' And when you go see it, it's like this unbelievable masterpiece, and I'm hoping people feel the same about the film."

Hayward, who makes her film debut after taking a temporary leave of absence from The Royal Ballet for the duration of shooting, agrees.

"Knowing what we would look like as cats, that was a bit of a mystery.

"We tried to imagine it as much as possible, but seeing yourself as a cat for the first time is pretty incredible.

"It's very strange to see yourself without your human ears; your cat ears are in a very different place and you have a tail ... I mean, that's really cool!"

Cats is released in UK cinemas now.