LOS ANGELES: Kristen Stewart instantly took up the offer to play Joan Jett in 'The Runaways' but later had difficulty singing in the movie as she doesn't consider herself a ''performer''.

Kristen Stewart had difficulty singing when portraying Joan Jett in 'The Runaways'.

The 19-year-old actress instantly knew she wanted the role of rocker Joan in the movie, but had problems with the singing aspect of the film.

She revealed: "I said that I wanted to do it as soon as someone said, 'There's a Joan Jett movie, and they want you to play her.'

"You do a movie for a million reasons, and I realised once I got into it that there was a lot more to Joan than I knew about.

"The most difficult thing was the performances, because I'm not a performer. The singing stuff, you learn so much about yourself. I don't know how to be more specific about it."

Her co-star in the movie, 16-year-old Dakota Fanning - who plays fellow Runaways band member Cherie Curie - says her and Kristen have become very close since filming.

She told America's OK! magazine: "She's become one of my best friends. We're really close. I think it's really great working with your friends, and to be completely open with someone and to have no defence mechanism, you know what I mean? Just completely open with that person. I love that - especially for this film."

Previously Kirsten has revealed the pair did have issues on set when they had to kiss, because Dakota was under age.

She said: "She was 15, and I wasn't allowed to grope her. I'm actually not kidding, there are major restrictions that I don't remember from when I was younger."

'The Runaways' is released in the US on April.