The actress explained that in order to prepare for My Week with Marilyn, she attempted to imitate the screen icon's body rhythm and famously breathy voice.
"I just tried to make her presence gradually and unobtrusively fill my life and my house. I tried to build it, so it didn't feel like work or study that I had to do, just a slow seeping in," Williams recalled to USA Today.
She added: "I started to try and mimic what I saw and what I heard privately in my house, and I started to take that a little bit out in the world, try something in the post office or the grocery store."
Williams further explained that playing Monroe at pivotal points in her life was "the hardest thing [she's] ever done".
"Most days it was an act of courage to stay on set, stay in the chair, and not flee," the star admitted.
Williams has commented that she didn't diet for My Week with Marilyn because Monroe was famously voluptuous.
My Week with Marilyn opens on Wednesday (November 23) in US cinemas and on Friday (November 25) in UK cinemas.
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Watch the trailer to My Week with Marilyn below: