In an effort to take on the tragic royal's facial features, the 44-year-old decided to alter the physical structure of her face for the role, reports femalefirst.co.uk.
"I didn't just want to appear like her, it's also about the tone of her voice and the way she moved her face. It's completely opposite of how I move my face. I move my face on the right-hand side and she moves hers to the left," Empire magazine quoted the actress as saying.
"I had to walk around with a cocktail stick in my mouth to paralyse the right-hand side of my face for weeks," she added.
The actress also spent weeks trying to perfect Diana's accent, confessing she was "very nervous" about authentically playing the character.