LOS ANGELES (Reuters) - British actor Idris Elba will be the first to say that he doesn't look like Nelson Mandela.

But in playing the anti-apartheid leader and former president of South Africa in the biopic Mandela: Long Walk to Freedom, he figured that nailing his physical presence would go a long way to portraying the man.

Elba, best known for his roles in the HBO television series The Wire and action films such as 2012's Prometheus, had to look past the elder statesman Mandela that everyone knows and find physical clues to the younger man, a lawyer who inspired many to join the fight against South Africa's all-white rule.

"What motivated people when they saw him as a young man was his presence, that he was a really commanding presence in the room and you wanted to follow him if he said 'Let's go,'" said Elba, 41 years old and 6 feet 3 inches tall.

The film, based on Mandela's 1994 autobiography Long Walk to Freedom, is an independent South African production directed by Britain's Justin Chadwick and distributed by The Weinstein Co. It opens in U.S. theaters on Friday.

Chadwick said there were few photographs of Mandela from that period, but in talking to people who knew him in his youth and boxed with him, he found Mandela was "this fireball of energy who loved life."

"I met him at 93 and, honestly, I walked into that room and you could feel the electricity coming from him. He has this energy," Chadwick said.

Mandela, now 95 and in poor health, gave South African producer Anant Singh the film rights to his story more than 15 years ago. Much of the difficulty in bringing the film to fruition was boiling down Mandela's story spanning six decades, including 27 years in prison, into a script for a two-hour plus film.

Chadwick and the producers decided to center the film on Mandela's fight against apartheid and the toll and sacrifice it took on his family. They wanted to show both the good and the bad, including Mandela's embrace of violence that led to a sentence of life in prison.

The film culminates with Mandela's inauguration as the first democratically elected president of South Africa after he urged his countrymen to reject violence following his release from prison.

'BIGGEST COMPLIMENT'

Chadwick, Elba and Naomie Harris, who plays Mandela's ex-wife, Winnie, were able to work off the memoir and the vast Nelson Mandela Foundation archive, but they relied heavily on conversations with family and friends to fill out their characters.

Harris, a fellow Brit who recently starred in the last James Bond film Skyfall, said that meeting Winnie Mandela was key to capturing her essence amid the polarized opinions about a woman both revered and vilified for her political fight.

"It really helped me to sit down and for her to say 'You portray me as you see fit. The only thing that I ask is that you do the job honestly and truthfully,'" Harris recounted.

Both Elba and Harris' performances have garnered praise from critics, more than the film itself. Variety's chief film critic Scott Foundas said that "Idris Elba gives a towering performance, a Mandela for the ages."

Elba never managed to meet Mandela, but said Singh did show the former president a final scene of the film, in which Elba playing Mandela is walking up a vale.

According to Elba, Singh said that Mandela asked him: "Anant, is that me? How did you get me to walk like that?"

"I thought that was the biggest compliment to me and what we were doing," said Elba. "He thought that image of me walking on the hill was him."

Elba calls the role the pinnacle of his career, one of his biggest challenges and a huge responsibility to be a part of a living legend's history.

"It represented the moment where I realized that with the right choice as an actor you can be quite powerful actually," he said.

He was further encouraged in his performance when he was able to convince the 600 or so South African extras in the crowd scenes in the township of Soweto - many whom had seen Mandela in the flesh.

"There's me pleading 'fight with me and act with me,'" Elba said.

"I am looking at an extra, I am looking in her eyes, she could just feel the emotion, I could sense her emotion while I was doing this scene. Those were scenes where I really felt like 'yeah man.'"

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