Julie Walters has told how 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 features 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 of the TV programme at Bafta in London that after studying footage of Ms Mowlam she worried she would not be able to portray her accurately.
Walters said: "I looked at acres of footage but I was very scared of it because she doesn't look anything like me, so I was very worried that physically I was nothing like her.
"And 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 bollocks, 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.
"She knew what her amazing talent was, connecting with the people, and she had a massive drive and ambition, and she was right in the end - history tells us that."
But writer Neil McKay said it had been wrongly reported that Ms Mowlam chose not to have life-saving surgery when she was diagnosed with a terminal brain tumour, so that she could keep working.
He said: "She couldn't have had an operation because the tumour was diffuse - that meant the edges of the cancerous tissue overlapped into the healthy tissue.
"It wasn't true that she opted for radical radiotherapy to keep her going.
"The words in the scene are literally the words that were said [to her doctor]. 'What can you do?', 'I can give you radiotherapy and steroids', 'Okay, I trust you.'"
© Press Association