Forget Oasis: there’s a more dynamic reunion in town. Gael García Bernal and Diego Luna, the lifelong friends who first paired up on screen to play oversexed teenagers in Alfonso Cuarón’s 2001 road movie Y Tu Mamá También, are back. Unlike the Gallagher brothers, Bernal and Luna never fell out or became estranged. For 20 years now, they have co-run a documentary film festival and two production companies in their native capital Mexico City. Their acting collaborations, though, have been few and far between, with only the soccer-based Rudo y Cursi and the eccentric Will Ferrell vehicle Casa de mi Padre to their name since Cuarón’s vibrant hit climaxed with them tumbling into bed together.

In 2010, the pair began sketching out a script idea about a prizefighter and his manager, which they felt would fit them like a boxing glove. But life and work kept getting in the way: Bernal has three children, Luna two. Their careers have also become more commercial. Bernal played a flamboyant conductor in four series of Mozart in the Jungle, and starred in the Marvel TV movie Werewolf By Night, while Luna reprised his role as the hero of Rogue One: A Star Wars Story in the TV spin-off Andor, which recently finished filming its second series.

After all that time, their punchy six-part comedy-drama La Máquina is finally here, with Bernal as Estéban, an ageing slugger coaxed back into the ring by Andy (Luna), his vain and reckless manager, who is menaced by sinister underworld forces. Most sports narratives are dominated by the athlete at the centre but the crises in La Máquina are evenly distributed. Droll editing – such as the sudden cut from Estéban on a romantic date to a shot of him snoring in bed later that evening – reminds us that he is no pollo primavera. Troubling hallucinations indicate that his problems aren’t purely physical. Meanwhile Andy, who has a borderline incestuous relationship with his mother, is chasing his lost youth with constant Botox top-ups, which have left him with what one character describes as a “condom-face”.

“I was surprised by how much Diego enjoyed the prosthetics,” grins the 45-year-old Bernal, lounging on the sofa in a London hotel suite and sporting an impressive badger-coloured beard. He is dressed today in a tight, chocolate-coloured jumper, black suit trousers with a fuzzy brown pin-stripe and black Chelsea boots.

Perched in a chair opposite, dressed all in black save for white trainers, is Luna, who is a year Bernal’s junior. “It’s unusual for me to change my appearance like that,” he says. “But I thought, ‘This is the film to really go there.’ Not just go there but to let myself go nuts. Andy’s having a reaction to the changes in communication in our time. With social media, all of us are being seen all the time. That wasn’t how things were when Andy started out, and he falls into the trap of wanting to be in the front line. He’s going through what we’re going through, except for him it is terrifying.”

What Bernal and Luna are going through is middle age. It’s a theme that didn’t occur to either of them when they first conceived La Máquina in their early 30s, but it emerged organically from the material. “The idea came out the more we talked about it,” says Bernal. “And the more we looked in the mirror,” laughs Luna.

“Exactly,” Bernal agrees. “Back in the day, I could get down to 65kg super-quick, no problem. Now it took a lot of broccoli and discipline to get in shape as Estéban. And I still couldn’t get there properly. It was so difficult.”

“There’s also a parallel between the characters and ourselves,” notes Luna. “As an actor, you’re constantly reflecting. When you are 20, you realise this might be the last time they ask you to play an 18-year-old. Nowadays I’m usually cast as the dad; I’m never again going to be telling stories as the son unless those stories start at a funeral. You’re always leaving something behind that you can’t go back to.”

This is said without rancour or regret. In fact, they both see ageing as a bringer of opportunity. “Acting gets better with age,” says Bernal. “Think of Anthony Hopkins and Kate Winslet. They were already good but they became better once they got older. You start to have more fun and lose your youthful insecurities. You understand it’s never been a competition or a race. It’s about what’s inside.”

Luna warms to the theme. “I’m capable of stuff I’ve always wanted to be capable of. Not just as an actor. I’m even a much better audience member! It’s like … I’m finally there. I have teenagers. I lost my dad. I understand things I didn’t get five or 10 years ago. It only becomes more interesting.”

Their children feel rather differently. “My teenage daughter is the one reacting to me getting old,” Luna explains. “I put on glasses when I read a book, and she goes, ‘What are you doing? They don’t look good on you, Dad.’ The other day I unplugged something, then went back 15 minutes later to unplug it again.” He laughs ruefully. “She reminded me of that mistake for the next week.”

Bernal is nodding in recognition. “My daughter doesn’t even like me to say, ‘Good morning.’ She’s like, ‘Dad, no. Stop. Please.’ But it’s great to surrender to that. I don’t fight it. You can’t win.”

The pair have known each other since infancy – their parents, who all worked in the arts, were friends before either of them came along. Bernal arrived 13 months ahead of his pal. “I was parking the car,” he jokes. “Yeah, he was making sure it was all right for me to come,” adds Luna.

They maintain that the first time they met was when one-year-old Bernal was taken to visit two-day-old Luna on the maternity ward. (“I think I ended up poking him in the face,” Bernal once said.) When they were nine and eight, they were part of a theatre company in Mexico City that invited its youngest members to devise and perform their own play. “Ours was called something like The Kidnapping of the Stars,” says Luna. I ask whether this was for friends and family and he looks comically affronted. “What do you mean? No, no. Real audiences.”

Bernal raises a finger. “In fact, we went on strike.”

“We quit!” Luna says. “The director was very mean to us, so we came out on stage after the 11am performance and told everyone before the 1pm show, ‘We are not going to perform. This is not fair.’”

“We were being mentally exploited,” says Bernal. “When we quit, it felt brave and empowering.”

Three weeks later, they noticed the show was still running without them. “There were other kids saying our lines in our costumes,” says Luna. “We realised, ‘Oh shit, that’s what happens.’ That was when we decided: next time, let’s produce it ourselves.”

He’s exaggerating, but only slightly. In fact, it was a few years after starring in Y Tu Mamá También that they set up their first production company. That film remains integral to their friendship, and in some ways, they’re always trying to channel the vitality they felt on set. It’s partly what La Máquina is about: how to retain a connection to one’s younger self without embarking on a doomed mission to recapture past glories.

“We were a very small crew on Y Tu Mamá También,” Bernal recalls. “When Diego and I were carrying the camera boxes across the beach, there was a moment where we looked at each other and it was like, ‘Doing cinema is fantastic, no?’ We had come more from theatre but here was something we didn’t inherit. Something that is ours.”

They are both a little misty-eyed today. “It was very special living through that with your closest friend, someone who had been part of your life since you were born,” marvels Luna. “We shared the feeling that it would have been very stupid not to do something with that.”

Mexico City is still home to both of them, though they don’t always see each other as much they might like. “We’re talking about a 44-year-old relationship so there is no one pattern,” says Luna. Bernal adds: “Life gets complicated, and the only way to do it is to plan ahead.” The last time they got together outside of work, he explains, was in June: election day in Mexico. “We had a table, bottles of wine, some food, a few other people there. It was a great feeling. I love election days. You feel like you’re in the control room.”

He turns to his friend. “Hey, we should go to more elections.”

“You mean elections in every country around the world?” says Luna.

“Yeah. Let’s do it. The election tour!”

• La Máquina is streaming in the UK on Disney+ from 9 October