“Sin City: A Dame to Kill For” was so green-screen heavy that cardboard boxes were used in place of cars on set, but one thing was completely genuine: Eva Green’s nudity.

Eva Green is the very model of a sexy siren in “Sin City: A Dame to Kill For.”

As the titular vixen in the noir thriller opening Friday, the actress had to embody a woman so alluring, she drives men to murder. Green did a great job — even the revealing poster of her Ava Lord in a see-through-top has been banned. But ticket-buyers will get a much better view.

Of course, as an actor you feel so silly being naked and you want to die, you just kind of become numb and you just do the scene,” Green, 34, tells the News. “And you know (director Robert Rodriguez) just has such taste and style that you are in good hands.

Jessica Alba is the dancer Nancy in “Sin City: A Dame to Kill For.”

“I really think [being nude in the film is] not gratuitous, especially for my character who uses her body as a weapon. She uses

Welcome back to Sin City, the boozy, bloody home of grifters and drifters, mobsters and molls. Like the 2005 original “Sin City,” the dark tales that make up the sequel are taken directly from the pages of Frank Miller’s iconic graphic novels.

But not everything is as black and white as it was the first time around. Now it’s the dames — like Rosario Dawson’s killer dominatrix — who dominate much of the R-rated flick’s action.

“There’s a lot of estrogen in this one,” says Jessica Alba, who reprises her role as the city’s erotic dancing muse, Nancy. “There’s very powerful women in this movie, and I think it’s so great because the misconception is that there can only be strong guys in this genre.”

A lot has changed for both Alba and her character in the nine years since the first “Sin City” was released. The last time around Nancy was a scared damsel in distress who danced away her insecurities for leering men in a bar — and the actress now says she felt just as self-conscious herself gyrating away.

Alba, 33, says she’s grown up a lot since, becoming a better actor and more importantly, a mother to two daughters , Honor Marie and Haven Garner. “I certainly feel that I’ve evolved as a person and an actress,” says Alba, “and I have [more] fearlessness as a performer after I had kids than I did prior to having kids.”

So she wanted Nancy, now an alcoholic out to avenge the murder of the only man she ever loved, to show the same type of growth, choreographing her sexy dances for months before cameras rolled and running around firing CGI crossbow bolts into the heads of her enemies.