The 30-year-old actress plays the iconic screen star in new movie 'My Week With Marilyn' and she admits the presence of the late Hollywood legend is looming over all aspects of her life.
She said: "I feel like we live together. At a certain point something else does take over. I don't quite feel myself these days."
The actress has also had 'lessons' in how to act like Monroe so she can give the best possible portrayal of the blonde beauty.
Williams added in an interview with the Daily Mail newspaper: "I knew I wouldn't be able to resist. But physically and vocally everything is different from me. I've kind of gone to school and had teachers to help me understand Marilyn, so I could project an essence of her. When I first approached the part, I thought that there were three, even four parts to Marilyn."
'My Week With Marilyn' will focus on the making of Monroe's 1957 film 'The Prince And The Showgirl' with Laurence Olivier.
It is based on a diary written by Colin Clark while he worked for Olivier - who directed, produced and starred in the film - as third assistant director.
The diary has already been published but details of one specific week were left out. This missing time frame will become the basis of the story.