The former Dawson's Creek star plays the iconic actress in My Week With Marilyn, and said that she soon came to understand what made her run so behind schedule.
She told CNN anchor Piers Morgan, 'I was curious about that because I didn't really identify with it. How can a person be so, in a way, disrespectful of the other actors and crew people that you're working with? And after about three days into week one I realised exactly how.
'When you spend three-and-a-half hours sitting in hair and makeup and then are required to transmit a kind of energy and vibration that makes people feel like you're sort of existing to give them pleasure... to put all that out on a consistent basis is exhausting.
'By Thursday I had a pretty good sense of maybe one of the reasons that she wouldn't want to get out of bed and go to work.'