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“It never occurred to me that someday I would be fighting for a part in which I played a stripper,” says Jessica Alba, whose first major role was as the genetically enhanced title character in James Cameron’s short-lived action-sci-fi TV series, “Dark Angel. “But my agent got a hold of the script for ‘Sin City,’ and I was like, ‘Oh yeah, I want to play her.’ “

“Sin City” is based on a series of graphic novels by the comic book crowd’s own super hero, Frank Miller, and co-directed by Miller and Robert Rodriguez.

Alba plays Nancy Callahan, one of the unofficial citizens of Old Town, a corner of the corrupt, brutal, crime-ridden Basin City–a.k.a. Sin City–that the police and the bad guys have declared a no-combat zone. Nearly a decade earlier, Nancy, a little girl, was rescued from unthinkable peril by a cop, played by Bruce Willis, and when he re-enters her life during another showdown with evil, she can’t deny her love for him, despite the age difference.

“She’s the sweetheart of the story,” Alba says. Miller has described her as an “angel in hell, sweet and pure.”

When Alba read the script, she says, she had no idea it was based on comic books, which she immediately went out and bought, and which she says made her even more excited about getting involved with the movie.

“I had to audition for the part, and I was most worried about Frank,” Alba says. “Believe me, there were a lot bigger names than me who were interested in the part. The first thing Frank said to me was, ‘Can you dance?’ … He had no idea who I was, so when he told Robert that he thought I was exactly what he imagined as Nancy, I was pretty thrilled.”

Alba says that while she was initially worried about taking her clothes off for “Sin City,” most of those fears evaporated when she took a fact-finding research crawl through strip clubs in Los Angeles.

“It’s not all that difficult; it’s all play pretend for the customers. Just like acting. You show them some skin, they show you the money,” she says.

What really concerned her, Alba says, was doing love scenes with Willis, who is 50. Alba is 23. “We were both really nervous about it,” Alba says. “Maybe him more than me, because he obviously doesn’t want to come off like some kind of perv. I was just grateful I didn’t have to get naked and roll around in bed. … That would have been way too embarrassing.”