updated 11/14/2011 AT 8:00 AM ET
•originally published 11/14/2011 AT 2:20 PM ET
She added a few pounds, had the blonde wig, and spent hours in makeup each day to play Marilyn Monroe in the upcoming film My Week with Marilyn. But Michelle Williams had to learn to move like the Hollywood legend, too.
“I wasn’t watching what I ate, let’s put it that way,” Williams, 31, told PEOPLE Sunday, at the film’s New York premiere, about her physical transformation, which also included some helpful wardrobe tricks that helped her jiggle a bit more.
“The costume designer and Michelle came up with some great stuff,” said director Simon Curtis. “But an awful lot is Williams’s understanding of the way Marilyn walked, and the way Williams herself took on the wiggle.”
“She did all the hard work, knew how to dance, shimmy,” added Kenneth Branagh, who plays Sir Laurence Olivier in the film.