“We’ve no time to sit and dither, while her withers wither with her” is arguably the greatest single line in Into the Woods – and emblematic of everything that is wonderful about composer and lyricist Stephen Sondheim. The very line fell into the lap of British actress Tracey Ullman, who plays the role of “Jack’s Mother” in the enchanting Disney musical.

“I don’t have an enormous amount to sing in the movie, but that was one of the classic Sondheim lines, and a lot of fans would say, ‘oh – you get the line!’” she told us, when we had the good fortune of sitting down with her to discuss this memorable project.

Adapted from a triumphant Broadway production by Chicago director Rob Marshall, this twisted fairytale intertwines the songs masterfully, as much of the narrative takes place within the musical numbers.

“You don’t stop and sing and entertain, it’s just the story throughout,” Ullman said. “It’s a real privilege to do it, it’s like Shakespeare singing. If you could qualify to be an interpreter of Sondheim and be good at it then it’s kind of thrilling. It has to be an extension of the scene, when you just sing for the hell of it it is awful. It’s embarrassing. So it’s finding that moment when you burst into song to make it flow. Sondheim wrote the lyrics to West Side Story. The worrying thing is, who is next? He’s the last of that incredible generation.”

Into the Woods may not be the most familiar play this side of the Atlantic, but is taught to American students at such a young age – including Ullman’s son, who put on the production at middle school.

“Everyone in America knows this musical because they did it at school, but I didn’t know it,” she admitted. “But my son was in school in America and did a production of it and he played Jack. Of course he was the best Jack ever and should have played it in the movie but I didn’t make a fuss [laughs]. So I watched it dutifully for ten nights and it really got in my brain and when I knew they were making a film I thought that if there was any way I could be a part of it that would be great.”

However rather than have her actual son take on the role (he’s now 23, to be fair….) the part of Jack fell to the Daniel Huttlestone, and Ullman admits she had a wonderful time working with the talented young actor.

“I loved Daniel, he is the coolest kid. Not one of those on his machines, talking to other kids, he liked adults. It wasn’t like ‘oh God I’m in this film with this boring old woman’ he just really talked to me. Me and Emily (Blunt) just think he’s great. Really funny and will go on to have a wonderful career. I just transferred my love for my son straight to Daniel and he just accepted it, and from the second I met him I just loved being his mum.

“Plus, having a real child playing Jack was good – when you see productions it’s usually a 37 year old man. Red Riding Hood is menopausal. So it was lovely to have real children in the film.”

Another co-star Ullman loved working with was Meryl Streep, who plays the Witch. As old friends who go back a long way, Ullman admitted she had some fun winding up crew members – particularly after James Corden had already convinced them all Streep had been sacked as part of a prank.

“I’ve known Meryl so long, that I would say to the crew, ‘she’s a nightmare, a pain in the arse. She might be a living legend but she’s boring. It’s all about her’. People would be so shocked that anybody was trashing Meryl. We used to pretend to have a row. But she’s the best thing, she keeps it real on every level and is just there for the work. No ego. She’s the best, and she sets the tone.”

Ullman also worked closely with another talented performer – though rather than be a multi-Oscar winner, it was a cow.

“I had a real cow, and I worked with my cow every day. She was beautiful, like a dog, I’d scratch her,” she said. Don’t eat them, she was wonderful! I got so fond of her, she was so clever. I’m crazy about animals so that was a joy. I would come home at night smelling of cow though, covered in straw. I loved it.”

That sense of authenticity extended throughout the set, as Ullman recalls such a brilliantly real experience, where an actual, built set was opted in favour of using CGI and green screen.

“It was so real,” she said. “The woods we built on the stage, it smelt like real woods, and there were trees and there was moss and there were birds, living on the set. Watch out, you’ll be gone in a few weeks! There wasn’t a lot of green screen of effects in this film, so it felt very real actually. There were a few moments where we had to step back and pretend Meryl had disappeared.”

Finally, we couldn’t help but ask whether Ullman had given her character a name – as the majority of characters are nameless – such as the Baker, the Baker’s Wife. So rather than simply be called “Jack’s Mother” we wanted to know if she had come up with anything different.

“Well James (Corden) and Emily (Blunt) called themselves Shaun and Brenda. James used to give us all names. He thought I would be a Pauline. As for my back story, well I don’t know what happened to her husband. Imagine it was cow related.”

Into the Woods is out in cinemas now.