Americans know Julie Walters as Ron Weasley's mom in the Harry Potter movies and as Billy Elliot's scrappy ballet teacher. Next up, she drops the maternal routine and strips down with Helen Mirren in Calendar Girls (opening Dec. 19). Yes, the 53-year-old British actress appears nude — although tastefully so — in the true story of some fiftysomething Englishwomen who pose for a cheesecake calendar in order to raise cash for charity. Did she hit the gym to prepare for this daunting task?

"I knew as soon as the script came that [nudity] was inherent in it, somewhere," Walters recalls. "I thought, 'What is this going to be? And what are they going to see?' Because I hadn't done any exercise in a couple of years. And I'm not exactly honed and toned!

"You can see my bottom," she cheekily adds. "So it made me think about [my body] a lot more. I'd think every so often, 'I can't eat that' or 'I'd better not eat now.' I did think that, but not hugely."

Thankfully, after finishing Girls, some forgivingly baggy costumes awaited Walters on the set of Harry Potter and the Prisoner of Azkaban (due out June 2004). And thanks to the new director — Alfonso Cuar&#243n — the wardrobe was even comfier than usual. "We had a French costume designer [Jany Temime]," she sighs happily. "The Weasleys look so chic [now]. I've still got my poncho, but it hangs in a different way. It has got these beads that pull it down. Just gorgeous."

Walters hates to keep secrets, so she gladly previews the third Harry Potter installment. Well, at least her part in it. "In [The Prisoner of Azkaban], I've only got a couple of lines," she laughs. "I think 'Hello!' is one of them. 'Harry, good to see ya.' I think that might be another one. But it is lovely going back each year to do it. It is really lovely."