GIVEN his movie star status, Idris Elba's used to people reacting in unexpected ways when they see him, but being saluted on the set of his latest movie was a new experience.

"The truth of the matter is, the extras had been going to boot camp three weeks before I even got there, so they were practically soldiers," explains the 43-year-old who plays Commandant, the rising war lord of a West African militia, in the harrowing Beasts Of No Nation.

"I show up and there's a natural respect for me as an actor, but then I'm playing the commander and we were sitting in tents in the jungle and we were an army. So every time they saw me, [it was] 'Yes commander'," he explains, doing a salute.

"I'd say, 'It's all right guys, I'm just going to the toilet, relax, chill out'. But it was a very sweet, warming, engaging process."

The movie is based on the 2005 novel of the same name by Uzodinma Iweala, a sociologist and physician who grew up in the United States but spent time in Nigeria, where his parents were born.

The tale brings to life the gripping story of Agu (newcomer Abraham Attah), a child soldier torn from his family to fight in the civil war, and who's taken under the wing of the Commandant.

"It's quite common knowledge that there are child soldiers, but the actualities of it, I think, are missing for us," notes Elba from behind sunglasses.

"We've never really got to the underbelly of who these people are, who their leaders are and what happens between them. So there was a real pull to create a cinematic journey that takes you into this world with a close lens, watching what a young person goes through to adapt to war, is the heartbeat of the film."

The director Cary Fukunaga, who also adapted the book for the big screen, and whose previous credits include the acclaimed 2011 adaptation of Jane Eyre starring Michael Fassbender and the first series of True Detective with Matthew McConaughey and Woody Harrelson, immediately sparked to the idea of Elba tackling the difficult role.

"It made sense because Idris is such a force of nature," explains the 38-year-old film-maker. "He's one of those guys who, even in real life, when he walks into the room, you feel that commanding presence, that gravitas. But I truly didn't think we could get him for this film."

As it turned out, Elba, who came to the fore playing the enterprising drug lord Russell 'Stringer' Bell in The Wire and earned a Golden Globe nomination for his portrayal of Nelson Mandela in Mandela: Long Walk To Freedom, was already a fan of the novel.

"I'd love for film audiences to gain what the book gives you: a deeper understanding of how child soldiers have come to be a resource for armies across the world," notes the London-born actor, who also co-produced the film.

"The use of children in war is steeped in very complex legacies, and Beasts Of No Nation reflects that. There could've been a very sentimental version of this film, but Cary steered away from that."

The Commandant might be a brutal war criminal, but Elba had no interest in offering up a one-dimensional villain.

"My approach was to humanise him," reveals the father-of-two, whose parents are from Sierra Leone and Ghana.

"I've got to make him human, I've got to make him watchable for two hours and [for people] not to absolutely hate him."

As part of his research he looked into "the common denominator among warlords", which, he says, is charisma.

"They're all charismatic, all have a commanding presence. Whether you hate them or love them, you'll find yourself compelled to listen to them. They might be talking absolute nonsense, but you find yourself compelled by it."

His preparation was a sobering experience.

"The objective is to numb these children, to detach them from their pain receptors, their emotions, and it was tough to look at.

"The Commandant also riles the kids up, continually reminding them that the government army destroyed their families and villages, and now they've been left alone. He tells them, 'You can fight back', and that's very seductive."

A "normally jovial guy", Fukunaga recalls how Elba started to take on a "guerilla-like alpha presence" during filming, saying: "It was intimidating. He really came to live the role, and it's not an easy role to live in."

The actor, who will be returning to the small screen as the maverick detective Luther, revels in variety in his work.

"Luther is a dark show but he's not a dark character, while Shere Khan [the tiger he voices in the upcoming Jungle Book adaptation] is," he explains. "And that's range, it shows variation and makes my life fun if I can play a good guy and a bad guy."

Elba was recently at the centre of a media storm following a comment by Anthony Horowitz, the author of the latest James Bond novel, saying he was "too street" to play 007. The writer later apologised — but what did the actor make of it all?

"You know what? I'm working on a film at the moment so I didn't see the [papers], it was the aftermath I got," says Elba.

"I pay no attention to it to be honest, because people who are offended by whatever he had to say, I honestly... [he waves his arm over his head, gesturing how he let it all go]."

Beasts Of No Nation opens across the UK in Curzon Cinemas tomorrow and airs on Netflix on Friday, October 16