They are sitting on the sofa opposite me; they appear close, friendly, at ease. Our conversation should be upbeat – a celebration of a father and son who have just been collaborating professionally. But as I switch on my tape recorder, we all know that a cloud hangs over this family team. And that cloud is Charlie Sheen.

The duo on the sofa are Charlie's dad, Martin Sheen, and his big brother, Emilio Estevez. And even if you only rarely watch television, and never read the tabloids, you can't have failed to register the tragic, relentless one-man circus (or should that be horror show?) that is Charlie, star of Two and A Half Men until he was sacked earlier this month, reputedly the highest-paid actor on the planet, now imploding before our eyes, his drug-fuelled, alcohol-soaked spiral of self-destruction unravelling grotesquely on an almost daily basis. How does it feel – how can it feel – to have Charlie in your family?

It seems almost unfair to pose the question. I feel bad, reminding his 70-year-old nice-guy dad (surely the best president the US never had in his portrayal of Josiah Bartlet in The West Wing) and his sensible-looking 48-year-old older brother (whose credits as a writer and director include Bobby, the story of the other Kennedy assassination) about the middle-aged delinquent back home. Especially as what they really want to talk about is their new film, which – coincidentally – is all about a lost son. For a moment, Martin looks unbearably sad; Emilio raises his eyes to the ceiling, and keeps them there. "OK," says Martin, in an "I-knew-this-must-be-coming" kind of a way. "We've had some counselling. I'm in Al-Anon – do you know it? – it's for people with relatives or friends who are alcoholics or drug addicts. You have to stay focused ... "

He quotes a version of the serenity prayer, much valued in Alcoholics Anonymous. "I accept my responsibility to change the things I can change and accept the things that I cannot, and I beg for the wisdom to know the difference."

More than the next father with an A-list son going spectacularly off the rails, Martin knows what he's talking about. After all, 30 years ago he was the hard drinking famous actor pressing the self-destruct button. He even had a heart attack at 36, while he was filming Apocalypse Now. More than 80 films later, and the personification of clean, sober living, he's surely one of the best-placed people alive to know about coming back from the brink. And of course if the "like father, like son" epithet relates to the current situation with Charlie, there's a chance it could also relate to a turnaround.

Emilio offers a Hollywood insight on the subject: "I take the Michael Corleone approach to family," he says. "I haven't decided whether Charlie is Sonny or Fredo. And Fredo had to go for a little boat ride, and Michael had to organise that … " Yikes: Michael Corleone did for Fredo on the boat ride in The Godfather: Part II. Surely Emilio doesn't mean he's about to do Charlie in? "I absolutely believe," Martin says, "that he'll come through it. I believe – like St Thérèse of Lisieux – that everything is grace. You don't see it or understand it, but when you begin to reach for the sacred ... everything is grace."

Reach for the sacred? Yes, says Martin: that's what Charlie is doing right now. Like everyone who uses drugs or alcohol, Martin believes, Charlie is searching, and his quest is for real meaning in his life. "Anyone who is using is looking for a transcendent experience. How often do people [under the influence of drugs] say 'I saw God'? It's an effort at transcendence. "

As his reference to St Thérèse suggests, what Martin is describing isn't only what he hopes will happen to Charlie – it's what happened to him. After Apocalypse Now and his heart attack, there was no further down to go: so Martin went up, right to the top. He found God, the God of his Catholic childhood: and he's never lost him again (at one point he mentions that he has just been to Mass at St Patrick's church in Soho Square on this visit to London).

Catholicism is writ large in the new movie too. Filmed largely in northern Spain, it tells the story of a fortysomething man (played by Emilio, in a cameo role) who dies while walking the Camino – The Way of St James, the ancient pilgrimage route across Europe to the saint's grave at Santiago de Compostela in Galicia. His father (played by Martin) travels to Europe to collect the body and ends up completing the walk on his son's behalf.

For both men, filming The Way was an experience steeped in family memories. The reason this father and son have different surnames is that Martin was born Ramón Estevez, the son of a Spanish father and an Irish mother, both of whom emigrated to Ohio in the 1930s. When he moved to New York in his early 20s, hoping to make it as an actor, he realised his name wasn't exactly helpful. "It's difficult enough trying to get started as an actor, and my name kept throwing people – they couldn't pronounce it. I thought, 'I've got enough problems, I can't be bogged down with this name.'"

Instead, he took the names of two people he found inspirational: one was Fulton J Sheen, the American archbishop – "He was remarkable: his delivery was like that of a Shakespearean actor" – and the other, Robert Dale Martin, a casting director at CBS.

So Martin Sheen was born – or reborn – but you sense, talking to him now, that he has always felt a little guilty about chucking in his Spanish heritage. But that's not how he sees it at all. "Estevez is still my name ... I never changed it officially," he says, digging his official ID card out of his pocket as proof. Emilio, his eldest child, his other son Ramòn, and daughter Renée, all chose to keep their father's real name … only the notorious Charlie, born Carlos Estevez, has adopted his father's stage surname, and anglicised his first name.

But the Spanish roots go deep, and part of the reason this film was made was to celebrate that. In fact, says Emilio, the irony is that, though in the movie it's Martin who's lost his son to the Camino, in real life it's he, Emilio, who has had that experience, though not in a tragic sense. His own son, Taylor, 26 ("I'd just learned to tie my shoes when he was born"), went to Spain with Grandpa Martin to follow the way of St James eight years ago, met his now wife in one of the refugios where they stayed on the journey, and has been there ever since.

Talking to Martin and Emilio, it's clear that their Spanish family are a real part of their lives. "I first went to Spain in 1969. I was working in Italy, and I took Emilio and Ramón and we went to see where my father had come from," says Martin. "We arrived in the middle of the night and we met my uncle Matias and his wife Joaquina and my other uncle, Lorenzo, and they put us to bed in the bed my father was born in! It was an extraordinary experience."

To this day, says Emilio, the whole of Galicia is crawling with Estevez relatives. "You'll be driving down the road and my aunt will say, 'that's your third cousin' as someone walks by, and then 'that's your second cousin' when you see someone else."

The Way is dedicated to Emilio's Spanish grandfather, Francisco.

So what was it like growing up in the US while his dad was an increasingly famous actor? "We travelled a lot – it was a given," says Emilio, looking at Martin. "We lived a very nomadic existence. I don't think I ever spent a full year in school because you'd go on auditions or your agent would call and they'd say, 'here's your assignment'.

"And more often than not, he'd say yes and we'd pack our bags and go somewhere, often overseas. That really informed the rest of our lives – that was our norm."

One of the most extraordinary things about Martin is that, throughout his own ups and downs, throughout the raising of four children, throughout the peaks and troughs of Hollywood fame and throughout the recent disasters over Charlie, he's been married to the same woman – Janet, with whom he celebrates his 50th wedding anniversary later this year. A Hollywood A-lister married to the same woman for five decades? How on earth have they managed it? "I haven't a clue," says Martin, laughing. "I really don't. I've never met a person with more integrity in my life, though. I honestly couldn't keep up with her. It's taken me a long time to be as even and direct with her as she has been, always, with me. And that's the relationship. I honestly don't have a clue who she is because when I get to the point where I've caught up with her, she's gone"

Did he ever imagine that all four of his kids would follow him into acting? Not at first, he says. "It was surprising to me. One time I thought Emilio had come to visit me on the set of a show, but it turned out that he was there because he had a part himself – his first part in a film. But when I saw him acting, I realised how talented he was. I was so relieved because each of my children was an individual talent. They didn't get anything by favour – nothing, not once. Emilio just reminded me the other day that he was up for a part ... "

"And I'd never been hired for anything," says Emilio, taking up the story. "I was reading for the director, and it was Sam Wanamaker, and I said 'Pop, you know this guy! Call him!' And I was furious when he wouldn't, and he said 'One day you'll understand why'."

"And I assume you have, since then?" quips Martin. Emilio retorts that he's never asked for another favour since that day. "But I've asked for a ton of them," says Martin. It's that shift in power, from one generation to the next, as the youngsters grow up and find themselves in the driving seat, and the parents who once called the shots gradually become the ones who must look for favour, not the other way round. Mind you, Martin is still an A-list actor – surely he'd still be working, even if he didn't have a director son to give him parts? Not so, he insists. "If this had been a normal Hollywood production, I'd have been the last guy in line to do it. My name doesn't sell a lot of tickets, not any more. I don't do big films these days ..." He trails off.

"Well, that could change after this one," says Emilio.

But the truth is, says Martin, that there's no one he would rather work with than Emilio anyway. "From my point of view there's nothing better. I'd work with him for the rest of my life, frankly."

The Way is released in the UK on 13 May