But Fanning wasn't intimidated by having to tap into her inner rock star; she really enjoyed it. "I wasn't dreading anything," she told MTV News while promoting the film at Sundance. "Actually, I was just really excited to do it. It's one of those things, like, you don't know how it's going to feel until you're actually there. And for me, [singing] 'Cherry Bomb' was what kind of summed the experience up for me."
But Stewart said that while Fanning has "got it," she wasn't quite as comfortable in the role of the rock icon — which is actually a good thing.
"I need to be thinking that this is the hardest job that I'll ever have to do, because I need to make myself nervous a little bit," Stewart explained of her acting process. "And I was intimidated by the singing part, even though it's not as much as what [Fanning] has to do. Joan has such a distinct sound, and I didn't want to sound stupid. I didn't want to sound like I didn't sound anything like her."
Stewart admitted meeting the real-life Runaways added a bit more pressure to the mix. "Having met them, it becomes such a different thing," she continued. "You're now responsible for the most important times of these people's lives that you've gotten to know and that you respect so much. And respect isn't even the right word for it."
The MTV Movies team was 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."
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