Eighteen months ago, Cary Fukunaga was Burbank’s golden boy. After laying award-winning groundwork with gritty thriller Sin Nombre (2009) and an acclaimed version of Jane Eyre starring Michael Fassbender (2011), Fukunaga directed a new crime serial called True Detective. It was universally adored. It spawned much talk about the future of telly, many column inches, a few knock-offs – and won the 36-year-old a heap of prizes. Life since has been less easy.

“After True Detective, I thought nothing could be as as hard as that, and Beasts eclipsed it,” he says, chugging green tea in a London hotel room. Beasts is Beasts of No Nation, a harrowing African drama that stars Idris Elba as a terrifying warlord of a squad of child soldiers. The shoot in Ghana involved malaria, extortion and near-death experiences.

“There just wasn’t enough money and enough time in a country that isn’t used to hosting a film production like ours,” says Fukunaga. “So things that you take for granted like food, transport and hotels became an issue.” We were losing stuff and things were getting stolen.”

Nor was it even easy to get to that stage. Despite Fukunaga’s impressive track record and Elba’s involvement, studios passed and the budget ended up coming courtesy of a bond company. “Every day I had a sinking feeling inside,” he says, “not knowing if we were going to have a movie by the end of it.”

Fukunaga sighs and smiles, relief still mixed with residual concern. Now 38, he was born and raised in California, and abandoned a dream of professional snowboarding in his mid-20s. His mother is a Swedish-American, his father was born in a Japanese internment camp during the second world war. “I think, in general, cinema is dominated by white, middle-class males,” he says. “That’s just fact. Part of that is an issue of diversity in terms of the content creators. I’m Japanese-American, but I’m probably perceived to be white.”

Fukunaga’s racial sensitivity is replicated in his movie. Beasts is uncompromising about the atrocities committed in some African civil wars – and the scars they leave on the perpetrators as well as the victims – but also with the ethnicity of its cast. We’re spared the kindly white UN worker whose viewpoint gives the audience an easy in.

Does he think a whiter cast might have meant a smoother road to securing finance? “Yes, without a doubt,” he says quickly. “The studios are in the business of making money, so if the numbers show that X subject delivers Y amount of money, then they are going to finance those projects. As long as the films are about this white person’s issue and that white person’s issue, they’re going to continue to make those films.”

Sin Nombre – an illegal migrant drama – also examined the politics within an impoverished community. As an outsider to both Mexico (where that film was set) and Ghana, he says he has worked hard to avoid fetishising “the other”.

“One of the biggest insults I got with Sin Nombre was someone calling it poverty porn. Whenever someone compared it to City of God – and I’m not criticising that film – I felt that my approach and my execution was not the same. City of God is a masterful execution of style and I was trying to be devoid of style. If the images were pretty, then my argument against that is that you have to make things beautiful by the very nature of framing the world. I frame it as I see it. There’s not a manipulation.”

That said, Beasts tested the limits of how much to show. Its imagery is shocking, but never explicit. Even its most horrifying moments “are not even nearly as awful as the reality”, he says. “As a society, we’ve been deprived of war imagery. I think if more people saw how gruesome war is, say in Syria, there would be a lot more global action to prevent it.”

Beasts is a daringly unsparing film. Its distribution, too, is a risk: a much-publicised purchase by Netflix, it premieres on that platform alongside a pared-back theatrical release. This is a canary-down-the-coal-mine strategy for a film seen as a major awards contender – and for a director known for his focus on cinematic imagery.

Yes, he says, he’s worried. “I am wary. It’s a complex relationship. I love the fact that Ted Sarandos [head of content acquisition at Netflix] is such a film lover and is so passionate about the film. But I also want it to be seen in cinemas by as many people as possible around the world. I know for a fact that the best way to watch the movie is in a cinema. It’s just the way it was made. It’s best for the sound, best for the picture and the best way to feel the energy in the room. The magnified emotional experience by sharing it with other people is best done in the cinema.”

Our time is up,, the green tea has kicked in and it’s time for the really tough question: what did he make of the second season of True Detective, the one audiences liked so much less, which was met mostly with scorn or shrugs. Fukanaga was, after all, a producer on that one, too. Oh, he says, he hasn’t seen it yet. Really? He insists so.

“The great thing about that show is that it’s an anthology, so every year will be different and next year has a chance to reinvent itself … and maybe it will have a warmer reception.”

Beasts of No Nation is released on Netflix today