HOLLYWOOD -- Jessica Alba has left the dark side, but she's still somebody's angel. The raven-haired beauty of TV's Dark Angel is now the bottle blond angel of Robert Rodriguez's comic book movie Sin City, which opens Friday.

She plays stripper Nancy Callahan who Sin City's creator Frank Miller calls "the purest, sweetest girl in Sin City. That's the reason she's up on stage performing and not down in the pit serving drinks."

When Miller drew the images of Nancy gyrating on stage for the male patrons of Katie's Club Pecos, it was quite some dance.

Alba, 23, had no idea how racy the images were until after she signed on to play Nancy.

"When I finally looked at the graphic novels I saw the pictures and I then found out that Nancy was a stripper and that she danced not only topless, but bottomless."

It was then Alba had a talk with her director.

"Robert said that the nudity was an option and, obviously, it would have been more authentic to the novels if we'd done it, but I felt that dancing around with a lasso and chaps was going to be sexy enough."

Alba explained to Rodriguez that "dancing around topless would be distracting for me and dancing bottomless would make my dad freak out."

Hence, the stripper Nancy keeps her clothes on for this visit to Sin City.

Another dicey issue for Alba was dealing with the fact her character is 19, but desires a relationship with Hartigan, the hard-boiled policeman played by Bruce Willis.

To the film's credit, Hartigan tries to fend off her advances and his desires by explaining he's old enough to be her grandfather.

"Nancy doesn't think of him as a father. She thinks of him as her knight-in-shining-armour."

Hartigan rescued Nancy from a serial child rapist when she was 11.

"I looked at it from the point of view that she waited until she was old enough to really be in love with him and have that relationship completely. I think Nancy always looked at Hartigan as her soul mate so I don't think the relationship is creepy at all."

Rodriguez admits he felt a little strange when he started directing Alba in her sensuous pole dancing sequence. "I first met Jessica when she auditioned for me for The Faculty. She was 16, cute and kind of dorky. Suddenly I'm asking her to be really sexy. I felt a little like Hartigan when he first encounters Nancy after those eight years and sees how much of a woman she has become."

Alba thought she was going to get the assistance of a choreographer, but recalls that Rodriguez "said for me just to feel it. He said he was just going to play some music and have me dance to it.

"He said that's what he'd got Salma Hayek to do for From Dusk to Dawn."

That was the wrong thing to tell Alba.

"That's the sexiest dance I've seen on camera ever. There hasn't been a sexier dance ever and he was asking me to think I could do the same."

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