The sweet serenity we associate with Brooke Shields flies out the basement-apartment window early on in Wonderful Town, right after she (as Ruth Sherwood) and Jennifer Hope Wills (as her sister Eileen) stake their claim to their grubby Greenwich Village digs.

With high heels locked into the footboard and heinie hoisted high in determination, she wrestles a reluctant sofa bed into rickety existence. It's the Looney Tunes version of unladylike, but count that fanny wave as a flag of defiance — a formal declaration that, at this performance glam be damned — and you'll see a newly minted, slightly demented Shields.

And damned if you don't! In the role that won Rosalind Russell a Tony in 1953 and Donna Murphy a nomination 51 years later, she stoops to conquer. "I'm saying, 'All right, I'm sacrificing all my dignity — just come along with me. I'm not afraid to look stupid so, please, let yourself laugh.' You have to give audiences permission. They're worried about being judged in a weird way, too. It's the crowd mentality. You have to let them in first and say, 'Look, I'm out here doing this, telling you it's okay, so why not, for two-and-a-half hours, let yourself have fun?' Hopefully, that sets the tone for the rest of the show."

The reason you haven't seen this sunny side of Shields before is simple: "I've never really been given the chance. It's such an eclectic part, so much in it, so much to do. I'm rarely asked to do this much, so when this opportunity came up, I jumped at it and took advantage of how many different people she gets to be."

Shields makes no secret that she gets what she gives — a good time. "This is the most fun I've ever had onstage. It's uplifting. The show goes by in five minutes for me. There's such an energy. It's constant. It doesn't let up."

This is Shields's third Broadway baptism-by-fire. She debuted as a replacement Rizzo in Grease! and did a turn as Cabaret's Sally Bowles. Grease, she now thinks, "felt more amateur. I felt very young when I look back and think about it. Cabaret was definitely like college, just like grad school. It was life-changing for me. It was over 9/11, so it was a very intense period, and I was in a really depressing play. I didn't realize how that was affecting me. There was a heaviness in my heart. But this [Wonderful Town] is buoyant."

"The face of the '80s" (Time magazine's designation) will turn 40 next year, so it's no wonder that visitors to Wonderful Town are startled to find a late-blooming funny face. "Everybody's been saying, 'I had no idea' and 'You did things up there I didn't think you could do.' It's a wonderful feeling to be able to surprise not only people but myself." Which is what happens when a girl jumps in with both feet.

"Feet, arms, legs, head..." she amplifies. To say nothing of the aforementioned fanny.