Jessica Alba has endured some ''very difficult'' dance training for her 'Sin City: A Dame to Kill For' role.

The 31-year-old star is to reprise her role as exotic dancer Nancy Callahan in the forthcoming thriller sequel to 2005's 'Sin City', and the screen beauty insists some of her alter-ego's routines are ''kind of crazy''.

She told Vulture magazine: ''I had to learn a bunch of dance sequences for it. I do a lot more dancing in this one.

''It was kind of crazy. It was very, very difficult, and you'll see why.''

While Jessica - who has two daughters, Honor, four, and 17-month-old Haven - took on additional training for her dancing scenes, she insisted on a body double for her stripping role.

A source recently said: ''She was crushed when filming began and she didn't have her original body back so she told producers she wanted a body double.

''Her body is incredible, but for her it's not good enough.''

Jessica will star alongside Josh Brolin, Joseph Gordon-Levitt, Rosario Dawson and Bruce Willis in the movie, which is due to be released on October 4, 2013.