Michelle Williams Pretended to Be Marilyn Monroe Even Away From Movie Set 

Marilyn Monroe is one of Hollywood’s most iconic screen stars, having won over the nation’s heart playing up her “dumb blonde” persona in films from “Gentleman Prefer Blondes” to “How to Marry a Millionaire” to “The Seven Year Itch.”

So in bringing to life the legendary actress in the highly-anticipated “My Week With Marilyn, actress Michelle Williams was surprised at just how witty Monroe really was.

“(People think) that maybe she wasn’t as bright as she actually was,” Williams told FOX411’s Pop Tarts at the movie’s Los Angeles premiere. “I just feel so lucky – I never thought I would wind up here, playing Marilyn.”

Based on a true story set in the early summer of 1956, “My Week With Marilyn” chronicles 23-year-old aspiring filmmaker Colin Clark’s first ever job as a lowly production hand on the set of “The Prince and the Showgirl.” That boring first job quickly turned into legend when Clark was witness to Monroe, fresh from her honeymoon with celebrated playwright Arthur Miller, setting foot on British soil for the first time to star in the film alongside British theater and film legend Sir Laurence Oliver.

Even when the cameras stopped rolling, Williams found herself afraid to step out of her Monroe character.

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“I tried to keep her voice, I tried to keep her walk, and I tried to keep as much close by as I could at all times,” Williams continued.

Convincing Williams to sign on to the intimidating role proved to be a challenge for director Simon Curtis.

“She’s a great actress playing a great icon. I was just desperate for her to say ‘yes,’” he said. “She was intrigued; she liked it, but had to go through her own process, which I completely understood, because it was a massive thing to take on… But when I finally saw her done up as Marilyn, it was a very exciting moment. The journey of watching her performance evolve on the set, day in and day out, was the most exciting moment of my career.”

Deidre Behar contributed to this report.