"My Week with Marilyn" is based on the true story of a young man who worked for Sir Laurence Olivier and documented what he witnessed during production of the 1957 film, "The Prince and the Showgirl."
Kenneth Branagh plays Oliver in the film and Julia Ormond plays his wife, "Gone with the Wind" star Vivien Leigh.
"The way that he's written it didn't fall into a sort of clichéd trap with Vivien just being bridled and jealous of Marilyn," Ormond said. "It was a woman who, two decades earlier, had similar experiences."
Dominic Cooper plays Monroe's business partner, Milton Greene. He was impressed by the film's leading lady.
"I felt really privileged to be a part of it and watch her make that transition," Cooper said. "It was incredible. I loved seeing her do it. She's magnificent.
Williams also did her own singing -- risky.
"It thrilled me and excited me to challenge myself in this way," Williams said. "I didn't think of it as a professional risk, I thought of it as a personal risk. And those I'm always willing to take."