Kristen Stewart has been everywhere at this year's Sundance Film Festival: on red carpets, in extensive sit-down interviews and on the screen in two movies, "Welcome to the Rileys" and "The Runaways." Both films showcase the actress in her most mature roles to date, and in each one she shows more than a bit of skin (beware of spoilers below).

In the last shot of "Runaways," in which she plays real-life rocker Joan Jett, Stewart is seen floating underwater without any clothes on. Was that really her?

"Yeah," she confirmed to MTV News. "But you can't see anything."

As Stewart explained, you can achieve things on a movie set that you might not be able to under ordinary circumstances. "There are things you are capable of doing under the guise [of acting]," she said. "You're so safe. Plus I was not naked. I was wearing stuff. I was wearing really, really, really unattractive nude moleskin or something. Really, really, really gross. In the movie, it's like, 'Oh, it's cool, this looks objective. She's submerged. She's lost in this world, this ocean.' "

In "Rileys," too, Stewart appears onscreen both in skimpy outfits and, once or twice, without clothes on, though the images remain decidedly in the PG-13 realm. She plays a teenage stripper and prostitute, and the scenes in which Stewart bares her skin are played less like sexy moments and more as melancholy examples of the degradation her character encounters. In other words, the scenes make sense for the story line.

The actress, however, wasn't so sure that her oceanic nude scene in "Runways" made sense. But under the direction of Floria Sigismondi, Stewart went for it.

"I was like, 'What is this? I'm in a bathtub and suddenly I'm in an ocean. I don't understand. OK, I'll lay in the water and look like a tool,' " she said.

The MTV Movies team is all over the 2010 Sundance Film Festival. Go to Sundance.MTV.com for exclusive clips, reviews, photos and interviews with everyone from Kristen Stewart and Ryan Reynolds to Ben Affleck and James Franco.

Check out everything we've got on "The Runaways" and "Welcome to the Rileys."

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