Naomi Watts found it tough to capture the voice of Princess Diana for the new biopic.

The 44-year-old actress - who plays the late Princess of Wales in 'Diana' - had two vocal coaches to help her perfect the ''posh'' voice of the people's princess.

She told ITV news: ''The voice was the toughest thing. Because it is posh - but she has that breath and she didn't have that stiff upper lip, she used a lot more of her facial expressions than that aristocratic way of Victorian ways.

''So putting that breath in to it and using her eyes and her mannerisms as well, that was all stuff that we remember well, her voice was something that we remember very well.''

Watts was so determined to perfect her role as Diana that she paralysed the right-side of her face by wondering around with a wooden cylindrical stick in her chops in order to change the way her mouth moved.

She previously said: ''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 the opposite of how I move my face. I move my face on the right-hand side and she moved her son the left.

''I had to walk around with a cocktail stick in my mouth to paralyse the right-hand side of my face for weeks.''