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 star plays the former secretary of state for Northern Ireland in the Channel 4 drama Mo.
The programme documents the late politician's battle with cancer as she fought for peace in Northern Ireland.
The actress said she studied "acres of footage" and was "very scared" because Ms Mowlam, who died in 2005, "doesn't look anything like me".
"She was a big woman, she's got a big open face, so I was very scared. But arrogantly, when I read the script, I just said, 'yes'."
Walters revealed her fears at a screening in London on Tuesday which was attended by members of Ms Mowlam's family, Labour MPs Harriet Harman and Adam Ingram, and former Home Secretary Charles Clarke.
"I looked at the footage and I thought, 'I can't play her. Will people remember her accurately?'
"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'."
Despite her misgivings, the star, whose other real-life roles have included Cynthia Payne and Mary Whitehouse, went on to shoot the two-hour drama and said 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 then-Prime Minister Tony Blair about how ill she was in order to keep her job.
When asked why she thought Ms Mowlam had not revealed the truth, 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."
The drama will be broadcast on Channel 4 on 31 January.