"At one point, she met me at Windsor Castle and she said, 'You are getting better, aren't you?'" Helena tells Entertainment Weekly, convinced the princess was referring to the actress' career.
Bonham Carter will play Margaret in the third and fourth seasons of "The Crown" as the show covers 1964 to 1978. Ben Daniels replaces Matthew Goode as the feisty royal's husband, Antony Armstrong-Jones.
"Everyone loves Claire Foy, so I have got the worst job in the world at the moment," she tells the magazine. "It's the same as any classical play you do - everyone will have already played that part before. The first week, I did feel myself trying to do Claire impressions: 'What would she have done?'"