Nelson Mandela's is no easy act to follow, the cast and crew of a new biopic on the anti-apartheid icon said Sunday at the film's world premiere.

Actor Idris Elba, who took on the role of South Africa's beloved, ailing former president, recalled that during filming several people "looked me in the eye and said, 'Do you understand the responsibility that you have here, pal?'"

"I wasn't expecting to be completely accepted as Madiba immediately for various reasons," the actor told reporters at the Toronto film festival using a term of endearment for Mandela.

"There was certainly a willingness for us to go for it, but know that you only have one chance, there's no messing about with this character and this story," he said.

"Mandela: Long Walk to Freedom" -- based on Mandela's autobiography tracing his life from his early years to the epic struggle he led to end South Africa's apartheid -- was 25 years in the making.

Producer Anant Singh said Mandela was still in prison when he first wrote to him, asking to tell his story on the big screen. Mandela sent a note back through a friend expressing surprise at the idea anyone would want to see a movie about his life.

"Here we are 25 years later, there have been several movies about him," Singh noted.

"And finally he chose me and granted me the rights against tough competition to 'Long Walk to Freedom,' his autobiography," the producer said with pride.

"I was very honored, but with that came a lot of responsibility."

Screenwriter William Nicholson said a big part of the story is how Mandela helped bring about a power shift without violence.

"I don't know of a single other example in history where an oppressed group has taken over without a bloodbath," the screenwriter said.

"And it's because Mandela was able to do that, through his power, to alleviate the fear of his enemies that we have a model in Mandela's life... that resonates across the world, that needs to resonate across the world."

Filming brought British director Justin Chadwick ("The First Grader," "The Other Boleyn Girl") and his brilliant cast -- Elba, Naomie Harris, Lindiwe Matshikiza, Riaad Moosa, Terry Pheto and Deon Lotz -- across South Africa.

Idris was even locked up at Robben Island prison for one night, in an effort to help him understand the hardships Mandela faced.

Chadwick said he spent time with Mandela's family, as well as with people on both sides of the conflict, in preparing to make the film.

Locals were used extensively in the production, including to build sets and design costumes, and as extras.

"Those crowds that you see when Idris and Naomie speak or Lindiwe speaks in front of the crowds, some of those people had actually been there at the time, seen the real Mandela, heard him," Chadwick said.

There are moments early in the film that show an unflattering side of the icon -- episodes of womanizing and once beating his wife -- but Mandela himself gave his blessing to portray him truthfully, Singh emphasized.

"We discussed the fact that we wanted to tell it all. He's the first to admit, 'I'm an everyman," with "'strengths and weaknesses,'" the producer said.

Elba said he "didn't want to deface Mr. Mandela in any way, but I also didn't want to portray him in a way that wasn't honest."

His portrayal emphasizes Mandela's personal sacrifices for the greater cause.

"We were able to make the political personal," said Nicholson, noting that Mandela, even at the moment of his greatest triumph, was actually very sad about missing many important moments in the lives of his family.

Late last year, Mandela was shown a few scenes from the film while it was still in production, on Singh's iPad.

"He said, 'Is this me?' and then he started laughing," said Singh. "That was to me that he endorsed Idris."