Sheridan Smith may be a critically acclaimed actress, but she feels like she is "totally blagging it through life".
The star, who won a Bafta for her performance as Great Train Robber Ronnie Biggs's wife, said while her "formally trained" co-stars will have scripts covered in writing, hers will be covered in "coffee stains and fag burns".
Smith said she has acquired self-doubt as she has become more successful.
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Speaking to the Sunday Telegraph's Stella magazine, she said: "It's just to do with my ability. When I'm working alongside someone like Dame Maggie Smith I can't help thinking, 'I don't deserve to be here'."
She added: "I spend most of the time feeling like I'm totally blagging it through life."
Smith also joked it is unlikely that she will go off to Hollywood to become a "juice-drinking jogger".
The actress is starring as Cilla Black in Cilla, an ITV drama series exploring the rise to fame of the Liverpudlian star.