The 44-year-old actress is set to play the late royal in new biopic 'Diana' and admits she was worried about whether she would be able to sound like her.
Speaking to Total Film magazine, she said: ''I was very nervous about the accent.
''She had a very distinct voice - she came from aristocracy so she was quite posh, but there was a warmth there, too. Her accent wasn't too 'fruity'.
''She also had this thing where the bottom half of her face didn't move that much.''
In one pivotal scene in the movie - which focuses on the final two years of Diana's life before her death in 1997 - Naomi recreates Diana's famous interview with BBC's 'Panorama' show, in which she said there were ''three of us'' in her marriage to Prince Charles.
The actress reveals she studied the interview rigorously in order to perfect the iconic moment in British history.
She added: ''I watched it pretty much every day during make-up; I went running with it playing in my iPod.
''It was probably the thing I was most committed to getting exactly right - the head moves, the eye moves, the hair - because everyone knows it so well. It's such an extraordinary piece of TV.''
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