Scifi Wire talks to Summer Glau. Slightly spoilery in the middle so here is the piece w/o the middle:
Glau: Firefly Secrets Revealed
Summer Glau, who reprises the role of the mysterious River in the upcoming Firefly movie, Serenity, told SCI FI Wire that fans will finally learn some of the secrets behind her character. In the short-lived Fox SF show Firefly, on which the movie is based, viewers knew only that River was a genius who had been surgically altered by mysterious forces for unknown reasons.
"We were setting up all the characters," Glau said in an interview. "We were going back into the past and trying to set up each storyline for each character. And my character was just getting started. And now, in the film, you're really going to see an explosion of what's really going on with River. It’s going to be exciting."
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Even so, Glau admitted that it took a moment to reacquaint herself with River and the Firefly universe, which is set 400 years in the future. "I was very nervous," she said. "I hadn't played River for a year and a half. I was so close to her in the beginning and through the whole series, and then when I came in for my first readthrough with the entire cast, I was shaking and sweating and I was really scared. And then after the first few days, it felt like I'd never left." Serenity, written and directed by Joss Whedon, opens Sept. 30. The film is being released by Universal Pictures, which is owned by NBC Universal, which also owns SCIFI.COM