Renée Zellweger: Making Movies Keeps Me Slim

By Shruti Dhalwala

12/04/2006 at 10:40 AM EST

Renée Zellweger, who showed up looking svelte at Sunday night's London premiere of her new movie, Miss Potter, credits her slim figure to her work.

She says that the 20-minute lunch breaks she was allotted while making another movie, Case 39, helped her stay fit.

"That's all we got on the set," the Cold Mountain Oscar winner tells PEOPLE. "At that point, I was like, you know, forget it. And running around … everywhere, that helps."

Zellweger, 37, joked that she finds out what diet she's on by reading media reports. "I'm like, oh, oh, I'm doing that diet now – that's fantastic! Good for me!"

As Peter Rabbit author Beatrix Potter, Zellweger is playing an English woman for the third time, having portrayed Bridget Jones twice. It's having an effect, too: "I understand (and) crave the beans and toast breakfast," she says.

But when it comes to living in the high-priced British capital, "I don't know if I can afford it," she told reporters. "I move around so much with work that I don't really live anywhere."

Next up for the actress: Leatherheads, with George Clooney. Shooting starts in January.

And how does she feel about working with PEOPLE's Sexiest Man Alive? "I'm scared," she admits. "I hear he's a terrible prankster and really had patience… Like when you least expect it."

As a result, "I'm going to be on alert, I'm never going to sleep, I'm sure of it. I'll be watching my back, George!"