'Twilight' leading lady Kristen Stewart spent the summer playing the role of rock-n-roll legend Joan Jett in the upcoming film 'The Runaways' -- now Jett reveals what she thought of Stewart's performance.

"Kristen was so into it, into the whole vibe of doing this," the rocker tells Billboard magazine.

"I think she felt a weight and a responsibility to interpret it correctly. She was really serious about it and was watching me and asking me all sorts of questions, from speech aspects to watching my body language, watching where I stood, watching my guitar playing. She really worked hard to get it right."

The film -- also starring Stewart's 'The Twilight Saga: New Moon' costar Dakota Fanning -- is about Jett's '70s teenage band called The Runaways. The movie is not set to hit theaters till some time next year.

Stewart is currently reported to be in Vancouver filming the third sequel in the 'Twilight' series.