"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.
Don Murray, who played opposite Monroe in Bus Stop in 1956, said Williams completely captures her likeness. "It's very, very accurate," he told PEOPLE. "Having spent 14 weeks with Marilyn, I know what she was really like. And Michelle really catches it."
What did Williams appreciate about the 1950s Monroe fashion? "I liked her kind of simplicity," she said. "She was so extraordinarily beautiful, but really quite unadorned."
Starring opposite Williams is Eddie Redmayne, 29, who plays Colin Clark, a go-fer on The Prince and the Showgirl, the 1957 film starring Monroe and Olivier. "I had seen him in Red, the play on Broadway. He won the Tony," Williams said of Redmayne. "And I immediately thought ... 'That's the boy, that's the one.' "
Williams herself has transformed fully back to herself since taking on the Monroe role. On Sunday, her famed pixie haircut appeared even shorter than usual. "I got a little trim about a week ago," she confided.
Lorenzo Agius / Weinstein Co.; Zuma