Matt Smith and Billie Piper

Billie Piper, who has gone from playing Rose Tyler to turning tricks as Showtime's Call Girl, has a message for Doctor Who fans: Relax.

Asked to weigh in on David Tennant's successor as the BBC series' Time Lord, Piper tells TVGuide.com she is a "big-time" fan of Matt Smith. "I love him."

Addressing the concern that Smith, at 26, is too young to effectively play the good Doctor, Piper notes, "The fans get so concerned about these things, and they should just relax. It's going to be fine. They're in good hands."

In fact, she argues that Smith's scant years will serve him well in the role. "He's a stirring actor, and he has the right energy," she says. "That's what the Doctor's is about, essentially — having that energy that you just can't understand."

Piper pooh-poohs talk that a female — perhaps even herself — was being eyed to helm the TARDIS. "David [Tennant] has joked about me doing it, but ... I don't think the Doctor should be a woman," she says. "It's like going, 'Let's make James Bond a woman.' It's a man's role."

Being a big fan of Smith's, might Piper be tempted to pay a return visit to Whoville? "I don't know," she hedges. "It's hard. Sometimes you just want to leave something behind, and then you see an episode and are like, 'God I really want to do it!' You change your mind all the time."