‘I’ve always dreamed of being in a boyband,” says director Eddie Sternberg, laughing – then quickly adding for clarity: “I’m kidding!” He’s talking about his debut feature, I Used to Be Famous. It’s the story of a washed-up former pop star played by Deadpool actor Ed Skrein, who gets his musical mojo back by jamming with a talented autistic drummer (played by newcomer Leo Long).

Actually, says Sternberg, he got the idea for the film 10 years ago when pop stars started doing comeback tours. “Blue, 5ive, Eternal – all these bands from when I was a kid.” Grabbing at a final 15 minutes of fame, they were only in their late 20s or early 30s. “I found that tragic, the idea of people having their peak in their teens or 20s, then trying to fill that void. And I love redemption stories.”

In 2015, he turned the idea into a short film, also called I Used to Be Famous. Now comes the feature film, commissioned by Netflix. It’s a cockle-warming heartfelt British movie in the style of The Full Monty and Billy Elliott, aiming to deliver all the feels.

“Feelgood doesn’t need to be a cuss word, you know,” says Skrein, grinning over Zoom from LA. He plays Vinnie D, who used to be a member of Britain’s biggest boyband. That was 20 years ago; since they split he has watched his former bandmate and rival become wildly successful, reaching Robbie Williams levels of fame. Vinnie’s solo career has reached the dizzy heights of busking around Peckham – using an ironing board as a keyboard stand.

Like the character he plays, Skrein has turned his life around more than once. “I’m going to be 40 next year,” he says, stroking his chin. “I’ve been on a couple of journeys.” Which is a bit of an understatement. After falling in with the wrong crowd, at 17 Skrein was stabbed in a knife attack that put him in hospital with a collapsed lung. Knuckling down, he enrolled on a BA in fine art at Central Saint Martins. He’d paint by day and hang out at the recording studio by night, where his mate, rapper Plan B (AKA Ben Drew), was writing an album.

Skrein started rapping himself. He dreamed of becoming an underground sensation, a breakout performer “with integrity”. Then came an epiphany in the cold light of his late 20s: “OK: this is the reality. There’s no money in underground music.” By that point he had a baby. “You can’t support a kid with it,” he laughs. He taught swimming to children at the local sports centre – a “legit, normal job”, which he loved. Still, there was a sense of failure: “The crushing feeling of not achieving what you had projected to the world that you were gonna do.”

Skrein started acting when Plan B cast him in a short film, which led to a role in the rapper’s 2012 movie Ill Manors. Since then, his career has rocketed, with parts in Game of Thrones, superhero flick Deadpool, the Transporter franchise and Barry Jenkins’ If Beale Street Could Talk.

Still, he could relate to Vinnie’s frustrations: the “compare and despair … I remember that time for myself,” he says. Besides, plenty of his mates in music never made it. “A lot of my peers didn’t achieve, didn’t buy houses and cars.” A lot of them now have mental health problems and addictions. “So, when you talk about drawing on these experiences, yeah.”

In the film, his character Vinnie gets a refresher lesson in creative integrity from a teenage drummer with autism who is applying to music school. That character, Stevie, is inspired by Sternberg’s cousin Saul Zur-Szpiro, a drummer who is autistic and has high support needs that require full-time care. Zur-Szpiro first picked up drumsticks at 10 – “at the time he didn’t have the strength to hold them”. Now he plays with rock band the AutistiX, which has a mix of autistic and non-autistic members.

Sternberg tells me about a charity gig the AutistiX played a few years ago, when they were unexpectedly joined on stage by Tom Jones. “I remember seeing that video and being blown away, knowing where Saul came from. Saul was someone who didn’t like crowds, didn’t like loud noises, and suddenly he’s performing in front of a thousand people with Tom Jones. For me that was the power of music, without wanting to sound cheesy.”

In the film, Sternberg wanted to create a representation of an autistic experience that felt real: “I think it’s important that neurodivergent people see a reflection of themselves authentically on screen.” He worked closely with the National Autistic Society, sending drafts of the screenplay he co-wrote with Zak Klein to the charity’s script reader, who is autistic – “making sure that they felt it was true.”

To find an actor to play Stevie, a nationwide callout went out to autistic and neurodivergent musicians and actors. “Our focus was on finding someone that had that lived experience,” says Sternberg. Films have often come under fire for hiring neurotypical actors in autistic roles. You get the impression the thought never crossed Sternberg’s mind. “I wouldn’t rule anything out in theory,” he says slowly, as if he was considering the idea for the first time. “But I was quite confident that we would find the right person who is neurodivergent. There’s plenty of neurodivergent and autistic actors – actors who have not had opportunities in the past.”

One of the hundreds of tapes that landed in his inbox came from musician Long – filmed by his mum at home in Hampshire. Long arrived at the audition in London with a banjo and an Irish bodhrán (drum) under his arm. “His personality was amazing,” remembers Sternberg. “He was also going through a relatively similar journey to Stevie. He was 19, pushing through into his 20s. He understood a lot of what Steve was going through. Also, I loved his story. He was nonverbal until he was nine years old.”

Over a video call, Long, now 21, describes himself as a “young neurodivergent bloke”. This is his first interview with a national newspaper, but he’s relaxed.

Did you have any anxieties about starring in a film, I ask? He grins ear to ear. Nope. “I think I’m pretty confident as an actor.” Quite rightly, he is proud of his achievements. “I punched a barrier so people can follow me.” What does he think is the message of the film? “I think it’s a brilliantly uplifting film about music bringing people together.”

Also on the call is acting coach Tricia Hitchcock, who supported him throughout the process. She works with Access All Areas, a theatre company for autistic and learning-disabled actors based in London. “It’s a creative coach; you take on acting technique as well as the person.” After their first meeting (“in a Pret a Manger” says Long), they did four half-day workshops together. “We started right at the beginning,” Hitchcock says. “Talking about what acting is.” Later they began to explore Stevie the character. “Who is Leo and who is Stevie? That is very important with neurodivergent people.”

I ask Long: what are the similarities between himself and his character? He beams. “Two similarities. One, they play music, drums. And two, wanting to leave home, being independent.” Happily, the film has brought him closer to the second goal. Long is speaking from Sheffield where he is rehearsing another role, this time in a theatre production of Much Ado About Nothing.

The actor who plays Stevie’s single mother, Amber, is The Walking Dead’s Eleanor Matsuura. It’s the kind of role that can be a bit of a turn-off: the anxious parent, all sunken eyes and worry lines. What appealed to Matsuura was that Amber wasn’t “the overprotective mother”. She is steely and emotionally attuned, an effective advocate for her son. Her concerns about Stevie are grounded in experience. The role got Matsuura thinking about the labels that get put on parents – “especially mums,” she says, pulling a face. “Like ‘helicopter’ and ‘overbearing’. Such harsh terms.”

I ask Sternberg how he modified working practices on the film set for Long and the other neurodivergent actors, who were hired to play members of a drumming club Stevie belongs to. (You can spot Zur-Szpiro, wearing an AutistiX T-shirt.) “The main thing was allotting time and safe spaces,” says Sternberg. “I wanted to create a nice, relaxed set.”

That said, neurodivergent actors were the “most efficient,” he adds. “They were ready to go. Saul especially, he loves repetition. So, because we’re doing so many takes, he really enjoyed that. He’d get more and more involved with each take.”

Has his cousin seen the film? “Yes. It was absolutely magical. He was leaning forward watching it the whole time. When it got to the end, he just turned to me and gave me a thumbs up. It gave me shivers.”

• I Used to Be Famous is released on 9 September in cinemas and on 16 September on Netflix.