London, England (CNN) -- He's played the president of the United States and he's played God twice, but that was nothing compared with the challenge of becoming Nelson Mandela, says actor Morgan Freeman.
"Mandela, you really have to know him," Freeman told CNN of his new role in Clint Eastwood's latest film "Invictus."
Freeman plays South Africa's first black president in the film and was recently nominated for a Best Actor Oscar for his portrayal.
"When you meet Mandela, you know you are in the presence of greatness. It is something that just emanates from him," he has said of the former South African leader.
The Oscar winner has lent his quiet authority to a number of acclaimed movies over the years, "Million Dollar Baby" and "The Shawshank Redemption" among them.
In "Invictus" he brings that screen presence to the start of Mandela's presidency, as he attempts to unite a bitterly divided post-apartheid South Africa in the run-up to the 1995 Rugby World Cup.
Capturing Mandela wasn't easy, Freeman told CNN: "You have to have watched him -- closely. Walk, talk, nuances of character, things like that," he explained.
Freeman researched his role by watching videotapes of the leader greeting people, talking to his staff, seeing how he dealt with different parts of his life, both public and personal.
As far as those close to Mandela are concerned, Freeman's hard work has paid off: "Everybody was over the moon about it -- his personal secretary, his wife [Graca Michel], his ex-wife [Winnie], his daughter, Zindzi, his grandson. They all said, 'You did it. That was wonderful,'" he said.
Even so, Freeman said he was "concerned" as he sat with the iconic leader, now 91 years old, to watch the film.
"I was sitting right next to him. He pointed at the screen and said 'I know that fella.' So, yeah, I think he liked it."
Freeman's relationship with Mandela goes back to 1994 when the leader named him as the actor he'd like to play him in a movie at a press conference for his memoir "Long Walk to Freedom."
"It sounds arrogant, but my thinking was, "Of course. Who else?" and I can do it. I know I can do it," Freeman told CNN. With this in mind, he bought the film rights to Mandela's memoir.
Freeman told Mandela that in order to do the part justice, they would have to spend time together.
Since then, the pair have met up in different parts of the world over the years, Freeman said.
Meanwhile, along with producing partner Lori McCreery, Freeman spent years unsuccessfully trying to work Mandela's long memoir into a film.
When journalist John Carlin sent him a proposal based on his book, "Playing the Enemy," Freeman immediately saw it was the story that could distill Mandela's quiet genius and achievement and communicate it to an audience.
Clint Eastwood came on board as director of the film (made by Warner Bros. Pictures, which, like CNN, is a unit of Time Warner) shortly after.
"It was very, very, very serendipitous," Freeman said. "Everything just came together in a very short time. In a heartbeat, by Hollywood standards."
Mandela's risky dream was to unite South Africa behind the national rugby team, the Springboks, who he wanted to win the World Cup.
In 1994, many black South Africans wanted to abolish the green and gold of the Afrikaners' beloved team, which they saw as a symbol of oppression.
Mandela realized that he must inspire the Springboks -- underdogs after not playing international rugby for many years -- to greatness beyond what they believed they could achieve.
He enlisted the help of Springboks team captain, Francois Pienaar (played by Matt Damon), and, for inspiration, shared with him the words of a William Ernest Henley poem, "Invictus," which ends with the words, "I am the master of my fate: I am the captain of my soul."
They were words Mandela recited to himself during the 27 years he spent in jail, imprisoned by a white government for fighting against apartheid. February 11 marks the 20th anniversary of the leader's release.
In June 1995, when the Springboks took the cup for real, the story of Mandela's inspired move made headlines across the world.
It is this Mandela -- the iron-willed risk-taker who united a nation and cemented his name as one of the great political leaders of the 20th century -- that Freeman captures.
After all, he said: "Playing God is not a challenge at all ... And playing the president, who's the president? He's just a guy. Mandela's not just a guy."
"Invictus" comes out in UK theaters on Friday 5 February.