When Dakota Fanning was approached to play a sexed-up singer from the Seventies, she knew exactly what to do.

“I didn’t know a lot about The Runaways or Cherie Currie when I got the script, and I looked online to see the videos of the Live in Japan tour and her performing Cherry Bomb, and I realized that I wanted to play her,” says Fanning, who knew of Joan Jett, but had little exposure to the electric era.

Jett is an executive producer of The Runaways (in theaters nationwide April 9), which is based on Currie’s memoir and chronicles the sharp rise and fall of the all-girl band.

Fanning embraces her first major adult role playing lead singer and sex kitten Currie opposite a leather-studded, chin-jutting young Jett, played by Kristen Stewart.

Fifteen, then and now

“A lot of people still think of me as young, and as the girl from I Am Sam, or whatever,” Fanning says on a sunny spring afternoon in her room at the Regency Hotel after a day of doing non-stop press. She’s plopped on a couch, dressed casually in skinny jeans, open-laced boots, a gray blazer layered over a graphic tee. Around her neck is a simple Tiffany gold key necklace.

“After this (film), I might be able to do different things,” Fanning, 16, says. “As you get older, there are roles you can no longer do and so many more things you can do.” Mentored on-set by Currie, on-screen Fanning dives deep into the pitfalls of fame: drinking, smoking (herbal cigarettes, she says), and thrusting sexuality full-force into the camera with each throaty cherry-bomb burst.

In person, Fanning is still the sweet, fresh-faced young actor behind past performances in I Am Sam, The Secret Life of Bees and Charlotte’s Web.

“I really liked that I was 15 (playing the role), and (Currie) was 15 and seeing what a different time I’ve grown up in and thinking, ‘I could probably never do this, I could probably never be on my own and on the road and performing in lingerie.’ ”

In a pivotal scene in the film, Fanning’s slender body is encased in Currie’s signature barely-there corset, worn originally on tour in Japan in 1977. Fanning keeps the costume at home.

“That corset is kind of how she expressed herself, and for Cherie, the way she expressed her sexuality was to wear that corset and to be the cherry bomb,” she says.

To beef up her vocal chops, Fanning worked with Currie’s original vocal coach.

“Then I was kind of thrown into the recording studio, and that’s what you hear in the movie and on the soundtrack,” Fanning says, noting The Runaways often recorded songs only a few times before releasing an album. “That was a little scary, but it actually worked out really well.”

With new opportunities have come new friendships. She and Stewart, called “Salt and Pepper” in The Runaways (a nod to their blond and black David Bowie-style haircuts), remain close.

“She’s become one of my best friends,” says Fanning, who first met Stewart after hopping on the Twilight franchise during New Moon to play bloodthirsty vampire Jane to Stewart’s innocent (and human) Bella. They reprise their roles in the next installment, Eclipse, scheduled for release in June.

No longer the little kid

Walking into New Moon felt as nerve-racking as the first day of school, Fanning says, but Stewart quickly brought her into the fold. “I’ve never really worked with a lot of young people in movies, and this is one of the first times I’ve worked really closely with someone that’s closer to my age than a father figure or mother figure.”

The two spent the first half of that New Moon day figuring each other out. “And then we were in a tent on the set, and it had a heater in it and we were alone, and we started talking and we instantly connected,” she says.

In a Runaways scene, the two kiss, something both approached with little fanfare. “I wasn’t even nervous,” Fanning says. “It just kind of happened. The way it’s written in the script, there’s no direction. It was something that happened, they were best friends. It was a different time back then.”

In her own life, Fanning has managed to build a surprisingly normal teenage life. She lives at home in Los Angeles with her parents, is crazy for Panda Express takeout and attends private school. Though she turned 16 in February, she cops to not having a learner’s driving permit yet.

“There are some days where I’m like, I gotta drive, I gotta get out of here, I should be able to drive myself, and there are other days where I’m like, “Mom, it’s OK, you can just drive me forever,’ ” she says with a laugh.

Of all her classes, she’s into psychology, and with college on her to-do list, she has a career in mind that emulates Jodie Foster’s.

After promoting The Runaways, Fanning will trade press calls for roll call, swapping corsets and vampires for pop quizzes and a cheerleading uniform.

“I thought it was something, you’re going to high school, maybe I’ll do cheerleading. It’s something fun and completely normal and a good experience to have. And if I ever have to play a cheerleader,” she says with a grin, “I’ll know what it’s like.”