When Kelvin Harrison Jr, 25, was working on his lead role in Waves, his manager asked him if he wanted to pull out. “There was a fear,” he says, about whether he and the film’s writer-director Trey Edward Shults could pull off its ferocious, narrative-shifting central event.

That tour-de-force moment, which we can’t get into without breaking the law of spoilers, would alter the direction of the film entirely, abruptly switching its lead from Harrison to fellow newcomer Taylor Russell, and transforming a joyful tale of young romance into a bruising one of violence and its consequences. If Waves is a film of two halves, then this jaw-dropping scene, lasting just a few minutes, is its crucial pivot. “Even A24 was scared!” says Harrison. (That’s A24 – the risk-taking production company behind Moonlight and Shults’s second film, It Comes at Night.) “But I was like: ‘No! I believe in Trey.’”

Waves, Shults’s third feature film, follows a seemingly ordinary middle-class family in Florida. There’s a stepmum and dad, a doctor (Renée Elise Goldsberry) and construction business owner (Sterling K Brown), and their two children – the shy and polite daughter, Emily (played by Russell), and the eldest son, star wrestling athlete Tyler (played by Harrison). The film plays with colour palette and aspect ratio, which, combined with a sleek, bassy soundtrack from Trent Reznor and Atticus Ross, captures the rush of teenage abandon. But it takes a dark turn when, in order to live up to the hopes of his domineering father, and to keep wrestling with a damaged shoulder, Tyler becomes dependent on painkillers. What follows is a bracing exploration of how one bad act can send shockwaves far and wide.

Waves is unusual in its depiction of black lives, rejecting the typical urban-and-struggling narrative and focusing instead on the middle classes, with many of the tropes of a white American psychodrama. Here, race isn’t an explicit focus, even though the undertones are there (“We are not afforded the luxury of being average,” says Sterling K Brown’s character in one poignant scene). It’s a unique and nuanced characterisation – but its writer, Trey Edward Shults, is not black.

A white writer creating black characters “has been done wrong a lot” reflects Harrison, but Waves’s characters benefit from Shults listening to his cast. Harrison describes the eight months he spent working with Shults while he developed the script. “We’d talk about our relationships with our fathers, our romantic relationships and the young men that we were at 18. I talked about friends that were athletes and what a scholarship meant.” (Harrison went to high school with NFL player Odell Beckham Jr.) And they spoke about black exceptionalism, and the pressure to live up to the achievements of parents.

“It’s exhausting!” he says. “My dad is a classically trained saxophonist-turned-jazz musician, my mum’s a jazz vocalist, both of my cousins are nominated for Grammys this year.” The expectation was that Harrison would follow the same musical path. He describes his life full of “after-school practices, Saturday rehearsals, Sunday church performances, and three jazz camps every summer … My dad was setting me up for the future, In white homes, they supply their kids with their family business to set them up for success, and in my family [music] was what they knew how to succeed.” he says. After years of secret auditions and acting classes, Harrison told his father he was quitting music, college and leaving the country. He’d landed a role in the TV drama StartUp, shot largely in Puerto Rico. “[Dad] came to visit me once but he kept saying I should go back to school.” And now, after seeing Waves, which recently landed Harrison a Bafta rising star nomination? “He’s gotten on the bandwagon.”

If Harrison is the star of Waves’ first half, then Taylor Russell takes control in the second, in a heart-wrenching performance that tenderly explores coming of age. It’s a role that Russell saw some of herself in. “[My character] is a quiet girl who easily goes undetected,” she says. “She has a lot more responsibility because her parents don’t worry about her. I get that. I’m the only girl and I had so much freedom to do whatever I wanted because my parents were focused on my brothers.”

Facebook Twitter Pinterest Table manners... Taylor Russell and Lucas Hedges in Waves. Photograph: A24

Fittingly, Russell follows in the footsteps of her actor father, who raised Russell with her mother (“a crafty creative person”) in Canada with “not a lot of money or resources”.” How do her parents feel about her role in Waves? “They’re very proud of me and I am too, even just for making it from Canada to America. I wouldn’t be able to do any of these things without all the sacrifices they made.”

In Waves, Russell’s character has her world torn asunder by her brother’s mental health and addiction, and she notes how such issues can repeat themselves across generations. “But those cycles have been broken [in my family]. I’m reaping the benefits of having a life that is bigger than my parents were able to”. Part of that bigger life is because of a changing world that is expanding our ideas of whose voices can be heard. “[Waves] is so different from anything I’ve done before because roles like this haven’t existed,” says Russell. “It’s incredible to be part of a generation in a time where we are championing change. Everybody’s craving it.”

She notes film director Alma Har’el’s criticism of the Golden Globes for failing to nominate a single female director or screenwriter last year. “We’re not in a time where anybody’s voice – person of colour or female – is going to be silenced. [But we do need] to get more stories from women and people of colour financed. Can that happen? I think so.”

When it comes to diverse characters on screen, there is always a question of authenticity. Waves offers a credible answer to the question of who can tell whose story: anyone, provided they listen closely and respectfully to those upon whose experiences it is based. For viewers, the end result is something unmissable, and for a promising new generation of actors such as Harrison and Russell, it’s something worth writing home about.

Waves is in cinemas from Friday 17 January