Michelle Williams likes running through fields with no shoes on.

The 31-year-old actress - who has six-year-old daughter Matilda from her relationship with late actor Australian Heath Ledger - is now playing troubled star Marilyn Monroe in movie 'My Week with Marilyn, and she admits has always liked her because she was a "simple dresser".

She told UK's Elle magazine: "Something I really appreciated about her is what a simple dresser she was. She's really, in her personal life, completely unadorned.

"Everything that she wore looked like she could take hr shoes off and run through a field. And I like that."

Michelle - who stars alongside Emma Watson and Dame Judi Dench in the film - also admits she was always more preoccupied with the "private" side of Marilyn as opposed to her Hollywood career.

She added: "I had always been more interested in the private Marilyn, and the unguarded Marilyn.

"Even as a young girl, my primary concern wasn't with this larger than life personality smiling back from the wall but with what was going on underneath."

Her love for the star - who died in 1962 - was deep enough for her to be very concerned about taking on the role.

She said: "I was so apprehensive. It was daunting living up to people's expectations as well as my own expectations."