She was previously married at 18 to popular DJ Chris Evans. Now 25, she's married to Laurence Fox, who plays Detective James Hathaway in "Inspector Lewis," airing on PBS' "Masterpiece Mystery" beginning June 22 (9 p.m. EDT).

Piper and Fox married on New Year's Eve, about a year after meeting while acting together in the play "Treats," and they recently moved from London to the country.

Piper has had recurring roles in the popular sci-fi series "Doctor Who," and has starred in several British made-for-TV movies. Yet never has the actress revealed more of herself than in the sexy "Call Girl."

The series is based on the Belle de Jour blog, which was written by a young woman who enjoys being a sex worker yet pretends to family and friends she's just Hannah, a legal secretary.

Doubts have been raised about whether Belle really exists or whether the blog was simply the fictitious concoction of some clever minds.

However, Piper says: "I met her and she was quite a character and she was the most useful part of my research."

"I started asking really simple, mundane questions — like `What's your favorite song?' `What kind of films do you watch?' Often those very simple details can be really useful. They tell you a lot about the human side of things," says Piper, explaining that because Belle's way of life was "so far removed from anything I know, I had to make her more human in my head."

She also consulted several other professional escorts and went to their apartments "where all these intimate moments take place" to see "step by step, how it's done."

How it's done, of course, meant strong language and nudity. But Piper counters criticism by saying that's just part of the acting challenge of honestly portraying the lifestyle.

Showtime is running the eight half-hour episodes uncut as part of an hourlong block with the fourth season of the hit series "Weeds."

Undaunted by the media criticism, Piper has begun filming a second series as Belle, and a third series has been ordered. But she may soon need a body double, she says, because she's pregnant.