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Perched on a couch in the Waldorf-Astoria hotel in New York City, Michelle Williams, looking demure in a vintage dress, is the first to admit she's no blonde bombshell. The idea of portraying screen legend Marilyn Monroe "kept me up at night," the petite actress says in between sips of hot tea. "I would say I spent something like nine months doing my homework." She read Monroe's diaries, letters and poems, pored over photographs and watched footage of the iconic sex symbol, who died of a drug overdose at age 36. "I was surprised to learn that she was really profoundly quite average," Williams says. "She had a normal walk, a normal speaking voice. The camera really transformed her."

Williams's own transformation in the new film My Week with Marilyn is winning her early Oscar buzz. A fiercely private single mom, Williams, 31, is best known for her vanity-free roles in indie movies. But Monroe wasn't so much of a stretch: The more she delved into the role, "the more I came to understand that she was an invention," says Williams, who spent three hours daily in the makeup chair and wore foam padding to enhance her curves. "Marilyn Monroe was the greatest character she ever played."

Growing up in Montana before moving to San Diego where she pursued acting, Williams even had a poster of the screen siren hanging on her bedroom wall. "The veneer was very seductive," she says. "Who wouldn't want to believe that this incredible goddess walked the earth?" Her 6-year-old daughter Matilda, whose father, Heath Ledger, died in 2008, has also become a fan. "Yeah, she knows Marilyn very well. She calls her 'Mara Monroe,'" says Williams. "She's seen a lot of her movies."

Since her debut on TV's Dawson's Creek in 1998, Williams has demonstrated her acting chops in films such as Brokeback Mountain (where she met costar Ledger) and last year's Blue Valentine, both of which won her Oscar nods. Now living in Brooklyn with Matilda, Williams strives for a daily balance between work and motherhood. "Kids just change so fast. It's painful to think about missing out," she says. "The other thing that I've learned is that the happiest houses are not the cleanest ones. As a single working parent, you're always asking, 'What's the most important thing right now?' I'd rather concentrate on my [daughter]."

Williams's latest role in the film Oz: The Great and Powerful is a hit with Matilda. "She comes to the set pretty much every day after school," the actress says. And unlike many of Williams's projects, "it's a real kids' movie. There's nothing that's going to frighten her, so it's been, I have to say, the best."

Even as she enters the blitz of award show season, Williams shrugs off praise about her turn as Monroe-with the exception of one particular pint-size opinion. "Matilda's actually been pretty sweet about it," the actress says. "Now when we see a picture of Marilyn, she'll say, 'Mommy, look! Look! It's you!'"

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