Where do you start with Shia LaBeouf? The former child star and teenage action hero, latter-day performance artist and Hollywood outcast vapes as a tune plays on his phone. The song is Our House by 1970s supergroup Crosby, Stills and Nash. (“A very, very, very fine house.”) He dances. “What do you want to listen to?” he asks the other actor in the room, Zack Gottsagen. “Pick a song.” Gottsagen, who has Down’s syndrome, shrugs and looks across at me. “He’s from the Guardian,” LaBeouf says in a tone that might be called sceptical. He switches off the music. “All right,” he says. “Let’s talk our shit.”

Ahead of this interview, I watch YouTube footage of LaBeouf’s 2017 arrest for public drunkenness in Savannah, Georgia. Physically, he looks much the same now, beard and T-shirt and worked-on upper arms. Then I watch The Peanut Butter Falcon, the film he was making at the time. It stars Gottsagen as a young man with Down’s syndrome who goes on the run from the retirement home he has been placed in by the state, joining a fugitive fisherman played by LaBeouf on an escape along the eastern seaboard. The result – charming, Mark Twain-ish – was the sleeper hit of the US summer. For Gottsagen, 34, the bustle of interviews and film festivals is something new. LaBeouf, 33, is less excited. “I’ve been on this rollercoaster several times,” he says. “So the only way for it to be fun is to do it with someone who has never been on it before.”

Gottsagen has been acting in amateur plays since he was a child, inspired by Grease and Hairspray, with a striking gift as a performer. In the film, he and LaBeouf play off each other beautifully. “I always wanted to do acting to express my feelings and also to show people my abilities,” he says. He met writer-directors Tyler Nilson and Mike Schwartz at a California film-making camp for actors with and without disabilities. The three resolved to make a feature to showcase Gottsagen – a herculean task given their lack of industry connections.

For a time, Nilson pitched the project while living in a tent in LA. When the idea did reach executives, the conversations ended when Nilson said that it would star an actual actor with Down’s. Finally, with a modest budget at last in place, the trio’s “proof-of-concept” video was sent to LaBeouf. It was April 2017, the actor en route to a performance art project – a month alone in a remote Lapland cabin, from where he watched the video over and over.

Facebook Twitter Pinterest Gottsagen and LaBeouf in The Peanut Butter Falcon. Photograph: Seth Johnson/Signature/Kobal/REX/Shutterstock

There was, LaBeouf says, a sense of truth to Gottsagen on screen he was desperate to act opposite. “I knew I’d be playing bass to his lead.” Did the novelty of a supporting role come as a relief? He is shaking his head before I’ve finished the question. “Not novelty. Novelty’s a shit word. There’s no novelty to Base jumping. Right? There’s no novelty to skydiving. Novelty is safe. This didn’t feel safe.” There is a long moment of eye contact. “This felt like lighting yourself on fire.”

I ask Gottsagen if he knew about his co-star before the film, and he nods and lists the hits of the chipper first act of the Shia LaBeouf story – the sitcom Even Stevens he starred in at 14, the Transformers movies, Indiana Jones and the Kingdom of the Crystal Skull. Beyond that, LaBeouf himself is acutely aware, the references get darker. The paper bag reading “I Am Not Famous Anymore” worn over his head at the premiere of Lars von Trier’s Nymphomaniac. The plagiarism of graphic novelist Daniel Clowes with his film Howard Cantour.com. The many run-ins with the police. The general air of hot mess.

My life had exploded again and I was in deep shame. This film saved me

La Beouf rarely deals with journalists any more. Before his last major interview, promoting the film McEnroe vs Borg in 2018 – naturally, he played John McEnroe – he spent weeks with his therapist preparing answers to potential questions. His publicist insisted that all the questions for this interview focus on The Peanut Butter Falcon. Among the subjects implied to be off limits is Honey Boy, another new movie that LaBeouf stars in and wrote, about his early life as a child actor. He plays his own father. There is talk of Oscars for script and performance.

LaBeouf’s answers tend to come as monologues. He describes returning to the US after a lonely month in Lapland to meet Gottsagen for the first time, the pair driven by van to Georgia to start preparing for the shoot. LaBeouf, skittish and talkative, was disarmed by his co-star’s silence. Eventually, they held hands. “And I don’t know if you’ve looked at this motherfucker’s hands, but these are the most beautiful hands, and something about holding them calmed all my fear.” The story is sweet, vividly told, and if transcribed in full, would take up half this piece.

Gottsagen’s memories of the drive are snappier. “I could tell Shia was a badass,” he says. But for him, the journey was still more profound, his film finally being made after a life of being told he could never be a professional actor. He wanted, he says, the industry to understand that disabled actors could be movie stars, too.

Although he has lived independently for several years, his mother Shelley remains a major presence in his life. Before filming began, LaBeouf says, she and he had a frank conversation. “On paper, I’m a fuck-up.” He says this matter-of-factly. “I’m not Tom Hanks. Search my name on the internet and you’re going to find a list of scary things. So it was a risk for them.” He wanted, he says, to spend as much time as possible with Gottsagen, mimicking their characters’ relationship. “For Zack, that was going to be uncomfortable, because here’s this dude who doesn’t want to go away. I don’t want to go to another room. I want to be in his room. I couldn’t ask certain actors for that. Certain actors are too professional.”

In the evenings during the shoot, the pair routinely hung out. They watched wrestling, much loved by Gottsagen, and scolded each other over their vices: Gottsagen ate too much ice-cream; LaBeouf drank too much gin. Alcohol has long been a problem.

At 4am on 8 July 2017, a drunk LaBeouf was arrested in Savannah, having initially approached a stranger for a cigarette. Evidence of how the situation escalated is widely available online, including bodycam footage from a black police officer who, the actor ranted at length, should not have been arresting “a white person who gives a fuck” in the age of Trump. LaBeouf was bailed later that morning, by which time word had reached the production, and everywhere else besides.

LaBeouf talks about his return to set in the present tense. “I can’t look anyone in the eye. And I can’t come to terms with it. And no one’s talking to me and I’m talking to no one, and I know my life has exploded again. And I am in deep shame.”

With the legal fallout still uncertain, Gottsagen sat on the floor opposite LaBeouf, to tell him he had jeopardised his one chance of starring in a film. Don’t blow this for me, he said. “I was mad,” Gottsagen says. “Angry. Frustrated.” Both men cried. “And I was worried about Shia doing the wrong things, being out in the middle of the night.”

When the shoot restarted, Nilson and Schwartz had the actors film a scene onboard a boat in which LaBeouf’s character is overcome by guilt. As they waited for “action” to be called, without knowing the camera was already rolling, Gottsagen reached out and lay his hand on his co-star’s back. LaBeouf crumpled into him. “That is not method acting bullshit,” he says. “To be given permission to come into this man’s life and the infractions still come – right then, I’m broken. That’s documentary.”

LaBeouf promised Gottsagen he would not drink for the rest of the shoot. Two years later, he is still sober. He later admitted the role in his case of white privilege (had he been arrested as a person of colour, other outcomes would have been likely). After filming came court-ordered rehab. There, a counsellor told him to write down the most significant events in his life story. Steeped in movies, he did so in script form, then sent that script to his friend, director Alma Har’el.

The result became Honey Boy, titled after his father’s pet name for him. Jeffrey LaBeouf was a Vietnam veteran and former heroin addict who is resentfully on his son’s payroll, the pre-teen Shia a jobbing actor on low-rent TV shows. A poor kid breadwinning in the space where a childhood is meant to go, without the boundaries that other 12-year-olds have, it makes the perfect recipe for an adulthood spent clashing with authority. The film is incredible – like a cartoon character turned flesh-and-blood in front of you, then telling you their innermost secrets.

By now, talking about it seems unavoidable. I ask LaBeouf which of his two new films feels more exposing. He answers without missing a beat. “This film is much more exposing than Honey Boy. Peanut Butter Falcon feels like real time. Like watching myself through a window. Honey Boy is a history. It hits hard, but it has novelty. It’s less dangerous. It’s much more something you put on a shelf.” Because he wrote it himself? Crafted it? “Yeah. It seems more naked, right? It’s not more naked. What it is, is a film about this clown mask everyone knows me for, that I’ve been wearing a very long time. To me, it’s not a revelation.

People tell me to stay away from Shia. They don't understand the real him. I tell them I won't

“Honey Boy explains my life up to this point,” he says. “This film saved my life. They’re different things.”

The tricksy nature of actors – suckers for a killer line – is not lost on LaBeouf. He also seems tired of make-believe. His career in studio movies has been put into deep freeze by his reputation, but much about his old life seems to go unmourned. “I hadn’t been in love with it for a long time,” he says. There was, he says, a lot of “novelty brotherhood”.

Facebook Twitter Pinterest LaBeouf’s bizarre headwear at the premiere of Nymphomaniac. Photograph: Picture Perfect/REX

Two years after leaving Georgia, he and Gottsagen are still frequently in touch. Outside the interview, they discuss attending the wrestling spectacular WrestleMania. LaBeouf has lately directed his energies towards Slauson Rec, the free acting school he co-founded in downtown LA; Gottsagen has taught other disabled actors for many years. LaBeouf talks about his honesty like a talisman.

“I look to him as an example. I know I come in defensive. I come in playing music. I say: ‘Here we go with the Guardian.’ I try to bring my world into the room. He doesn’t. You came in here and he was sitting quietly. He’s Paul Newman. So I look to him and I know I need to switch my life up.”

The two now have their arms around each other. Gottsagen tells LaBeouf he loves him. “I love you, too,” LaBeouf says.

He starts to speak again, but Gottsagen – having spent the past few minutes sitting being talked about – has his own monologue to deliver. “I have been changing Shia’s life,” he says. “That’s how I do things. And Shia has proved to himself he doesn’t have to act out. So I’m going to keep taking the chance of being close to him.” Every word is measured, deliberate. “A lot of people have come to me talking about Shia, and they always tell me: ‘Stay away from Shia.’ But I am not going to do that. My answer is: ‘No.’ Because people don’t understand the real Shia. And those people need to stop saying those things about Shia. And I don’t really care how people feel about that, because I can make my own decisions.”

The Peanut Butter Falcon is released today. See review, page 10