All-star Chicago cast

She is one of Hollywood's hottest leading ladies but no one has ever seen her like this - in the film adaptation of the smashhit musical Chicago. Catherine Zeta-Jones has gone back to her West End roots to take on the role of sexy, ruthless murderess Velma Kelley. And Rene Zellweger, better known as the pudgy and hapless Bridget Jones, has transformed herself into the muscular and hard-as-nails convict Roxie Hart. Both actresses wore wigs in the film, as well as putting themselves through a gruelling fitness regime to get in the best possible shape for the high-tempo dance routines.

Zeta-Jones might have had a small advantage over her co-star - she has had prior experience of how tough it can be in musicals, starring in 42nd Street in the West End at the age of 17 after paying her dues in other shows, including Bugsy Malone and Annie.

But Zellweger was a complete newcomer to musicals before Chicago began shooting. She had never even been in a school production, although she did try.

She said: "In college I couldn't get in a show. I tried out for Hair but that didn't work out. I went to see the show - that's the closest I ever came to musical theatre."

She added: "Learning a whole new medium of expression is so fulfilling in ways that I never imagined .. . to use your body in that way, to express yourself in that way . .. I shouldn't even try to explain it."

However, Zellweger admits that she was initially very worried by her Welsh co-star's on-stage expertise, saying: "I had heard that Catherine could sing and dance. People would say, 'You're not going to believe it'. Well, I walk in on my first day and she's in a lift singing All That Jazz and I was just dumbfounded.

"My jaw dropped to the floor. I didn't speak for 30 minutes. Just sat in the corner in awe and watched. It's not just singing and dancing, she belts it out.

"I'm like a little field mouse compared to her. Squeak, squeak, squeak. And the movement - I would stand a few steps back and watch her. I was learning, basically, to dance by watching."

Typically, for a Hollywood actress, Zeta-Jones has nothing but compliments for her co-star, saying: "Rene is amazing. She's so gung ho. I swear to God, we love each other.

"We're two very strong, very different personalities. But we were really in this together."

Zeta-Jones will not be able to attend the film's world premiere in Leicester Square on 8 December.

She is expecting her second baby in March and will stay in America with husband Michael Douglas.

However, Zellweger and co-stars Richard Gere, who plays lawyer Billy Flynn, and rap-singer Queen Latifah are attending.

Meanwhile Chicago the musical is celebrating its fifth anniversary at the Adelphi Theatre in the West End. Denise Van Outen, Ute Lemper, Alison Moyet and Claire Sweeney are among the stars who have wowed audiences with Kander and Ebb's classic show tunes. The anniversary party last night also celebrated the arrival of the show's new stars. Former EastEnders heartthrob Michael Greco is the new Billy Flynn and the role of butch lesbian warden Mama Morton is filled by TV presenter Gaby Roslin.

Roslin said: "I was very nervous before the show but the minute I went on stage it was 'there's the spotlight, there's the band - Oh my God'"

Claire Sweeney said: "This was the first time I've seen the show since I left and I loved it - it really is the best show in town." She said she had spoken to Zellweger about the film. "I met her when I was in the show and she was already working on the movie. But I didn't offer her any advice - I mean how can you give advice about acting to a star like her?"