Actress Julie Walters said she almost pulled out of playing Mo Mowlam because she feared she was not up to the job.
The 59-year-old Mamma Mia star -- who has played real-life roles including Cynthia Payne and Mary Whitehouse -- stars in new Channel 4 drama Mo, based on the former Secretary of State for Northern Ireland's battle with cancer as she fought for peace in Northern Ireland.
Walters revealed at a screening last night in London that after studying footage of Ms Mowlam she worried she would not be able to portray her accurately.
The actress said: "I was very scared of it because she doesn't look anything like me. I rang the agent and I said, 'You're going to have to get me out of it, I must be mad to think I can play this. 'It's like asking Daniel Craig to play Gerry Adams, it's just impossible.'
"And there was a big silence on the other end and then he said, 'With respect, that's b******s, get the wig and glasses on and get on with it'.
"But it was fabulous to do because it was a wonderful script, she was just alive in that script."
The drama documents how Ms Mowlam lied to Prime Minister Tony Blair about the severity of her condition to keep her job.
Asked why she thought Ms Mowlam had done this, Walters said: "Exactly why she said. First of all it would be political suicide, and secondly it was what fuelled her life, so it was like holding on to life."