WHY SHE'S ON OUR RADAR: The pillow-lipped former pop star and coolest actress in the U.K. sizzles as Belle, a high-priced escort, in Showtime's latest potboiler series, Secret Diary of a Call Girl.

HER SHTICK: Sexily unapologetic and clueless about the line between business and pleasure.

AS SEEN IN: Doctor Who, Mansfield Park

POP TART: Famous at 15 in Britain with her hit song "Because We Want To," and again at 18 for marrying the U.K.'s top DJ, Chris Evans (16 years her senior), Piper, 25, gets the public's front-page fascination with child stars gone bad. "But it's really hard when people start looking up to you and you have to suddenly become picture-perfect. That kind of pressure makes people go mad."

HAPPY HOOKER: "We're exposed to a side of prostitution that is very tragic—the drug-fueled, trafficking end of it. But to research this role and sit in front of the woman whose experiences this show is based on, a woman who is well-read and graceful, and has a degree—it challenged my initial prejudices."

LETTING IT ALL HANG OUT: "I knew what I was getting myself into. I couldn't turn up on the set and say, 'Oh, I don't feel like getting my tits out today.'"

HORSING AROUND: In one scene, Belle literally rides atop her middle-aged client. "I was OK because I was in my underwear, but this actor was exposed, wearing a saddle, pretending to be a horse. I don't know what that does to one's self-esteem."

HOMEWORK? "My husband [her second, actor Laurence Fox, son of James] hasn't benefited from my role. It's one thing when you're pretending to be a love machine. But if I had to do it in real life, I'd find myself absolutely hilarious."

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