Before his filmmaking career took off, Benny Safdie performed stand-up comedy under the persona of Ralph Handel, an alleged banker who did open mics after work. In a 2006 YouTube clip, Safdie, then 20 years old and dressed as an office drone, deliberately bombs a six-minute set that feels like an hour, never revealing his true identity.

“My goal with stand-up was that everyone in the room would go, ‘That’s a real person feeling and saying these things,’” Safdie tells me in late September, still in a suit, but only because it’s the day after a London movie premiere. “That’s what I said to Dwayne and Emily. I wanted them to reach a bar where you think those are real people up there.”

Safdie, who’s 39, is referring to Dwayne Johnson and Emily Blunt, the leads of his new film, The Smashing Machine. After directing Heaven Knows What, Good Time, and Uncut Gems with his 41-year-old brother Josh Safdie, Benny has gone solo: he wrote, directed, and edited The Smashing Machine on his own, winning the Silver Lion at the Venice Film Festival. A subversion of sports biopics, the A24 drama showcases Johnson as a three-dimensional actor who finally makes the most of his muscular physique, self-awareness and delicate pronunciation of dialogue. The Rock, through Safdie’s lo-fi lens, has jagged edges and fissures revealing fragility inside.

Safdie based his script on John Hyams’s 2002 documentary The Smashing Machine: The Life and Times of Extreme Fighter Mark Kerr. In 1997, Kerr (Johnson, transformed by prosthetics) is a champion MMA fighter who tells journalists he can’t imagine losing; still speaking like Johnson, he’s redolent of the actor’s optimism in junkets for the Fast & Furious franchise. The film then reveals Kerr’s downfall over the next three years: broken bones, lost teeth, hospitalisation, drug addiction, and a toxic romance with Dawn Staples (Blunt, dressed like Posh Spice).

Watching both films is a fascinating exercise in spotting what Safdie kept, discarded and reimagined. One of the biggest laughs in Safdie’s script is someone saying “yes” at a press conference when asked about face-kicking in new UFC regulations; in the doc, it cuts off after the question. “I wanted to respect the reality of what happened,” says Safdie. “I love the documentary. But I can change things. If a scene takes place in a small room, I can make it more filmic, and still keep the feeling.” Safdie compares it to sharing life moments with his therapist. “We put things that happened to me in the past on the table, we look at them, and then we put them back in my head. In that process of reimagining things, you learn about yourself.”

While a typical script would have Kerr rebounding from failure, Safdie sticks to reality: Kerr’s career crashes by 2000. More invention goes into the fights that take place outside of the documentary: not UFC matches, but verbal battles between Kerr and Staples. Outdoing Adam Driver punching a wall in Marriage Story, Kerr rips a door off its hinges; the wrestler could do something similar with his girlfriend’s neck, but he instead comforts her with his gigantic hands. It’s to Johnson’s credit – and perhaps decades of witnessing his carefully curated public persona – that the character’s warmness means you never actually fear for her safety. The door, yes, but not Dawn. “Mark puts his finger in Dawn’s face,” says Safdie. “I was aware of that threat. His emotions are running high. You don’t know what’s going to happen. She says, ‘Would it be easier if I punched you in the face?’ and she pushes him. It’s like: whoa.”

Between Uncut Gems and The Smashing Machine, Safdie acted for auteurs like Paul Thomas Anderson, Claire Denis, and Christopher Nolan. He also co-created, co-wrote, and co-starred in The Curse with Nathan Fielder. The show’s out-of-this-world finale has, in retrospect, taken a new turn after the latest season of The Rehearsal. “Nathan’s flying lessons were while we were editing The Curse,” Safdie clarifies. “But he’d always been obsessed with airline disasters.” So it was subconscious? “Possibly. From the beginning, we knew the show was about a supernatural event that happens in [Nathan’s character’s] head, and nobody believes him. We thought: what if something supernatural happens [at the end] and there’s proof it’s real, but nobody believes him?”

I made amazing movies with my brother, and I learned a lot in that process. Yes, I do argue with myself, but, at the same time, I don’t have to pull somebody else out of the room, and be like, ‘This is what I think you should do!’

In The Smashing Machine and The Curse, cinematographer Maceo Bishop shoots scenes from voyeuristic angles. Safdie compares The Curse to a hidden-camera prank show like Candid Camera, whereas on The Smashing Machine, there’s more tension: “It’s like they know we’re in the room, and you feel like we shouldn’t be allowed there.”

Josh Safdie once described the duo’s creative partnership as being “founded on arguing, artistically”. As a solo filmmaker, does Benny have to argue with himself? “There’s less of that, because I don’t have to have a conversation about it. I can just do things. I made amazing movies with my brother, and I learned a lot in that process. Yes, I do argue with myself, but, at the same time, I don’t have to pull somebody else out of the room, and be like, ‘This is what I think you should do!’”

Johnson, who purchased the rights to The Smashing Machine, contacted Benny Safdie as a potential director after watching Uncut Gems. The closest comparison, though, is Lenny Cooke, a 2013 basketball documentary by both Safdie brothers. The first two-thirds are 2001 footage of a promising athlete; the final third follows Cooke years later as a depressed has-been. Using camera trickery, it ends on the older Cooke addressing the younger version of himself in the same frame.

A similar clash between the past and present occurs in The Smashing Machine when the film jumps to 2025 with the real Mark Kerr playing himself. Safdie describes the cameo as a risk. “People were saying I couldn’t do it,” the director recalls. “But I said to Dwayne very early on, ‘I am so confident in what you’re going to do as a performer, that we can cut to the real guy directly after seeing you, and everybody’s going to see the same person.’ It was like I put it all on red, to be like, ‘It’s going to work!’”

The gamble paid off: Safdie’s next film, Lizard Music, an adaptation of a children’s book by Daniel Pinkwater, will star Johnson as Chicken Man, an eccentric human whose girlfriend is a 70-year-old chicken. “Hopefully you leave The Smashing Machine excited about life,” says Safdie. “I’ve got two kids. I want to do something like that for them. Lizard Music is a crazy story that will appeal to everybody from the age of 6 to 96.”

The Smashing Machine is out in cinemas on October 3.