Before we begin, Idris Elba asks for a moment. He paces around the room for a few seconds, with a kind of nervous, tired energy, his focus somewhere outside the walls of this hotel room.

His father recently died and, instead of finding some space and time to process that and come to terms with it, he is being flung across continents by the publicity machine for the biopic Mandela: Long Walk to Freedom, in which he plays the lead.

In 24 hours, Nelson Mandela will die, but neither of us know this, of course.

Elba settles down, yawns and draws a deep breath and he is back in the room, and he focuses on me, which is kind of great.

Director Justin Chadwick interviewed a lot of people during years of research before making Mandela: Long Walk to Freedom, asking Mandela's friends and family about their memories of him as a young man.

''Everybody talked about Mandela being a star. He would walk into a room and you would feel it,'' Chadwick says. ''Even when I met him, I could feel the energy coming off him.''

Elba has that, too. He has an invisible, magnetic pull, a sense of being the centre of any room he is in. And he knows it. He is a big flirt with the ladies, like Mandela was, and he has a boxer's build, like the young Mandela.

He is exactly the same height as Mandela, and he is charming and confident, but he is just an actor, just some guy who pretends to be other people for a living, being asked to portray one of the century's greatest men, from youth to old age.

He was shooting Prometheus when Chadwick offered him the role.

He hesitated. ''I couldn't understand what kind of film it could be where I could play this character,'' Elba says.

''I didn't want to mess it up. He's not a character that you're ever going to get forgiven if you mess it up. He's a real human being … and I wanted to do justice to him.''

It is much easier to play a bad guy than a good guy, Elba says. ''Everyone has the capability of being an asshole. But being a good guy, being earnest and being real, you have to take the audience along with you. They can sense when someone's being fake. The hardest parts for me were the parts where he was this big, genuine 'I am Mandela and I'm going to run the country'.''

Elba is an instinctive actor, Chadwick says. He inhabits a role and has to believe, to internalise, every moment of a scene. So somehow this British lad had to create in himself the feeling of suffering under, and rebelling against, apartheid South Africa.

There was something in his past he might have drawn on: he grew up in East London, in an area heavily populated by members of the fascist, racist National Front. ''It was tough,'' Elba says. ''I was 12, 14. I would just be walking to the shops and get a bunch of lads going, 'What are you doing you black c---'. Or I would be chased home from school.

''Later, when I got older, it was about fighting for yourself. I got into massive fights. I worked at this chicken shop right opposite a pub. The [patrons of] the pub used to come out and into the chicken shop. They were horrible lads. They used to disrespect us. One day it went off in there horribly.

''That was the sort of thing I went through. It wasn't like mass persecution. It was very much, 'You want to f--- with me, I'm going to f--- with you'. I didn't suffer racism, you know. I fought against it.''

Which is a pretty decent paraphrase of Mandela's thought process, as per his autobiography, as he decided passive resistance wouldn't be enough, and built the armed wing of the outlawed ANC to fight the violence and oppression of apartheid.

But this is all coincidence. Elba didn't draw on his past to play Mandela, apart from anything else because he didn't need to. ''South Africa's presence is enough,'' he says.

He went to Robben Island and spent a night in the bare cell that Mandela lived in, lying with his head touching one wall and his toes against the other.

''When I got let out in the morning, I was so f---ing angry,'' Elba says. ''What the f--- is this about? F--- these people. Let's go and make this f---ing revolutionary movie about Mandela.

''I definitely felt a 'real Idris' infuriation about being treated like that because I'm black.''

And then being in the scenes, being forced to strip in the rain in a prison yard with white prison guards shouting abuse. ''The history of being there … it feeds the whole thing,'' Elba says, leaning forward, energised and darkly glowing with the memory of the experience. ''Those actors were really good and they're talking to us like shit. That was fuel. It fuelled the scenes.

''I didn't have to ever sit back and think about my own life, because it was like I was in it.''

Chadwick says he obsessed over creating an ''immersive experience'' for the actors. Much of the film was shot on location, in the places where these historical events took place.

Most sets were 360-degree, with the camera in the middle and the scenes all around. On set, he even insisted that inside the drawers in houses you would find something ''right''.

The emotions he was filming were real. The extras were locals, fired up by their real lives and painful memories.

It never quite got out of control, but when filming a riot scene on the streets of Soweto, ''sometimes cars would go up or tyres would be lit'' in unscripted, impromptu anger, Chadwick says. To walk into that scene as Mandela ''took a certain type of actor, a brave person''.

At the premiere of the movie in South Africa, Elba was surrounded by Mandela's family and former colleagues: 700 South Africans who had lived the reality of this film.

''After the film finished, people clapped politely, then silence,'' he says. ''It was this moment. I just thought, 'Oh no, did they not like it?'''

Then he saw the tears in their eyes, and realised ''like'' was not the right word. At the after party, they let him know how they felt, and they told him which scenes he didn't quite nail and which scenes he did.

They thanked him and they embraced him, and Elba finally knew that he had done OK.

Elba never met Mandela. He didn't want to before the film, because ''it wouldn't be beneficial to either of us'', but, he tells me, he would like to.

''I just want to see him for one time, just see him for real,'' he says. ''Just to have that moment and look him in the eye and say, 'In some small way, Mr Mandela, I've helped maintain your legacy'. I don't want a pat on the back. I want to say it was an honour.''

A day later, Nelson Mandela is gone.

Mandela: Long Walk to Freedom opens on Thursday.