Michelle Williams in "My Week with Marilyn"

Laurence Cendrowicz/The Weinstein Company

Michelle Williams, seen here, stars in "My Week with Marilyn."

In an interview with CBS News correspondent Mo Rocca, Williams said when the star was playing Marilyn Monroe, she wasn't actually being herself.

When asked if Monroe was actually a character, Williams said, "I would say so, and it's beguiling to think maybe that's who she really was, because who wouldn't want to believe that that's possible? But you know, that's just the shell."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Dougray Scott and Michelle Williams in "My Week with Marilyn." Scott plays Monroe's husband, Arthur Miller, at the time the film takes place. Michelle Williams portrays Marilyn Monroe.

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Michelle Williams as Marilyn Monroe in "My Week with Marilyn."

Williams told CBS News, "Things that I've read (about Monroe) said she was actually quite, I hate to say it, but ordinary. I read something once that was so moving -- that her face could fall apart in between glances, and then (she could) put it back together -- that it was something she could rearrange in a matter of seconds."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Dougray Scott and Michelle Williams in "My Week with Marilyn." Scott plays Monroe's husband, Arthur Miller, at the time the film takes place. Michelle Williams portrays Marilyn Monroe.

Williams said Monroe felt best when she was in front of an audience. "The audience was the only home that she'd ever really known. And the only people that understood her were the audience -- not the industry that she'd given most of her life to, not the three attempted marriages, but the audience. And even from photos too, I think she sort of got a confirmation of her existence. No father would claim her. Her mother couldn't be consistent with her. And the photos and the audience were sort of proof that she belonged."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Derek Jacobi in "My Week with Marilyn." Jacobi plays Sir Own Morshead, the royal librarian at Windsor Castle.

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Eddie Redmayne in "My Week with Marilyn." Redmayne plays Colin Clark, an employee of Sir Laurence Olivier's, who documents the tense interaction between Olivier and Marilyn Monroe during production of the 1957 comedy "The Prince and the Showgirl."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

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Kenneth Branagh as Sir Laurence Olivier in "My Week with Marilyn."

Branagh told CBS News that Olivier swore off directing for 20 years, at least in part because of his experience with Monroe, who was often late or refused to perform when the muse didn't descend.

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Julia Ormond stars as "Gone with the Wind" megastar Vivien Leigh in "My Week with Marilyn."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Dominic Cooper plays Milton H. Greene, executive producer of "The Prince and the Showgirl," the 1957 film featured in "My Week with Marilyn."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

Emma Watson of "Harry Potter" fame as "Lucy" in "My Week with Marilyn."

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams

From left, Zo

Judi Dench in "My Week with Marilyn." Dench plays Dame Sybil Thorndike, a distinguished tragedy actress.

Michelle Williams, Kenneth Branagh talk Marilyn
Pictures: Monroe pictures on the auction block
Pictures: Holiday films 2011
Michelle Williams: Becoming Marilyn Monroe
Pictures: Michelle Williams