As his seminal 1980 play, True West, is revived in London, the influential playwright reflects on the diminishing global role of the US, and the deaths of his friends Philip Seymour Hoffman and Robin Williams

Sunday evening in Santa Fe and Sam Shepard and I are sitting at a downtown bar, drinking tequila and eating tacos. The light is low, the night warm and the conversation darts and dives while the bartender rattles the cocktail shaker and behind us the tables begin to fill. Already we have covered several pressing matters, including the merits of Chekhov ("I'm not crazy about him as a playwright… why are you going to bring a dead bird onstage?"), the qualities of greyhound piss ("like champagne" apparently), and the ancient Egyptian goddess Isis: "The way she turns into a bird! Unbelievable. You can't make that shit up."

But now our conversation has turned to the subject of True West, the play Shepard wrote in 1980, now revived at the Tricycle theatre in London. Directed by Phillip Breen and starring Eugene O'Hare and Alex Ferns, the production first appeared at the Glasgow Citizens theatre last year, earning much acclaim, not least from Shepard himself, who was instrumental in ensuring its London transfer.

"I think Phillip's production is great," he says this evening. "And the actors are terrific… You rely on great actors." He recalls one of the play's most notable stagings, in New York at the turn of the century, the two leads played by the late Philip Seymour Hoffman and John C Reilly, who alternated parts every so often to keep things lively.

Shepard saw Seymour Hoffman a week before he died of a heroin overdose in February and says he had no inkling anything was awry. "He was overweight, but he was overweight a lot," he says quietly. "And he was pretty tired. He said he was going to go back and take a nap… See, I don't think he meant to kill himself, I think he had some bad heroin. Though I didn't realise he was that much of a junkie."

He pauses. "I knew Robin [Williams] pretty well and Robin knew he wanted out – he had Parkinson's. The two guys were very similar in that they were both overwhelmed by their own thing. I know a lot of people who've died… who've taken their own lives," he continues after a moment of quiet. "But you know Patti [Smith], who's an old, old friend of mine, she wrote a review of the new Murakami book that appeared in the New York Times, and at the end of it she said, 'I don't want to kill myself, I want to see what happens.' And what a statement. I believe her."

For more than five decades, Shepard has been one of the most prominent and respected figures of American stage and screen. For some, he has been principally an actor – the star of Days of Heaven, The Right Stuff, Frances, where he first met Jessica Lange who would be his wife for nearly 30 years, and more recently Brothers, opposite Jake Gyllenhaal, and The Assassination of Jesse James… with Brad Pitt.

But for others, certainly himself, he is first and foremost a writer. He began writing for the stage in New York in the early 1960s, having dropped out of an agriculture degree, spurred on by reading the work of Samuel Beckett and by the desire for contemporary America to have a theatrical voice. "Back then, there was a dearth of American theatre," he explains. "There was nothing going on. American art was starving."

He wrote La Turista, Angel City, Cowboy Mouth, a collaboration with his one-time partner Patti Smith, among many others, before True West, Fool for Love, various short stories, sketches, essays and a screenplay for Wim Wenders's Paris, Texas. In 1979, he won the Pulitzer prize for his three-act play Buried Child.

Today, he divides his time between his farm in Kentucky and his home in New Mexico, where he holds a post at the Santa Fe Institute – one of several "highly accomplished, creative thinkers [appointed] to catalyse transdisciplinary collaboration, synthesise ideas and methods from many disciplines, and enhance, or even define, new fields of inquiry", according to the institute's literature.

"I go there every day," is how Shepard puts it, his tone hovering lightly. "It's kind of interesting – it's a thinktank situation, 95% of it is scientists. Me and Cormac [McCarthy] are the only two writers. Everybody else is a nuclear physicist. Which is cool, you know. But it leads to a lot of conversational dead-ends." He laughs wheezily – many of his sentences end this way, in a warm, chest-deep rasp.

At the moment, he is writing his first novel. "After six book collections, basically I thought, 'God, wouldn't it be so great to be able to sustain something?'" But he is hesitant to expand on plot or themes. "Errrhhh," he says, stickily. "I don't know how to explain it. I really don't. Hopefully it's a novel, but I have the hardest time sustaining prose. I feel like I'm a natural-born playwright but the prose thing has always mystified me. How to keep it going?" Another long chuckle. "How do people do it, for years and years? I've been working on this for 10 years!"

The institute has helped. In Kentucky, he would be tempted away from work, by his horses and his cattle, by the easy pleasure of riding a tractor around the farm. Here, he has a desk and the air of academic rigour radiated by an institution. "It's a great discipline," he says. "So I'm very content for that reason. I mean I produce… pages. Pages!" He looks faintly amazed. "Whether they're any good or not…"

The trick to balancing the demands of writing, acting, theatre, film and novels is simply "that I don't do them all at once. Not like Peter Ackroyd, he works on a history, a biography and a novel all at the same time, he just goes away for a day and does that," he says, astonished. "I read a long interview in the New York Times; they asked him, 'What do you do when you're not writing?' He said, 'Drink'." Shepard laughs, and raises a toast. "Slainte," he says. "Cheers."

Born in Illinois (though probably conceived, he points out, in Texas, where his father, an air force pilot, was stationed and where "my mother tracked him down and jumped the fence…"), Shepard was raised in California and lived variously in New York, California and London before New Mexico and Kentucky. Even now, at 70, his life is often led by the roaming nature of film-sets, most recently shooting in New Orleans and Florida.

Shepard and Patti Smith in a performance of their play Cowboy Mouth in 1971. Photograph: Gerard Malanga

Not surprisingly, an itinerant quality has imbued much of his work, a feeling that, as he says this evening, "I don't belong much anywhere". It has been writing, he says, that has been his mainstay, his home, and by way of illustration he tells me about travelling with Bob Dylan's Rolling Thunder revue in 1975: "It was pretty insane. Now that I look at it, it wasn't really, but I wasn't accustomed to transience, every second was all about movement. And I was glad to get back to a kind of constancy. Writing was the constancy."

He is inspired "not so much by landscapes but by its connections to the past", and talks of the pueblos that once covered New Mexico, the 2,000-year-old pottery you can kick up with your feet in the desert. His own connection to this area goes back a long way too: "I can remember going through this town when I was eight," he recalls. "On my way to Chicago. I remember being very alone. Very, very alone. Being stranded on the train in the middle of Indian country."

It's the kind of recollection that makes you consider the extent to which Shepard is, like Clint Eastwood, Aaron Copland or Frederic Remington, bound up in the mythology and narrative of the American west, his life plaited into his writing and his roles, and how much this has been part of his appeal.

He set True West in the suburban California made familiar to him by his mother's home in Pasadena. It focuses on a fierce sibling rivalry between Austin, an Ivy League-educated screenwriter, and his wayward brother, Lee, who claims to spend much of his time in the Mojave desert, making dubious ends meet. As the pair house-sit for their mother, they come into increasing conflict over the sale of a screenplay, and their contrasting lifestyles, their tussle set against the sound of crickets and coyotes, the death of house plants, the stealing of toasters.

In essence, the play explores the ideas of the insider and the outsider, identity, family and America's idea of itself, examining the point at which the new west of civilised, suburban America meets the wild and uncontained old west. And so it seems fitting to be sitting in a desert city with Shepard this evening, to listen to him take stock of American culture 34 years after True West was written.

Shepard's first example of an outsider was his father. He laughs with a frustration that has faded to fondness. "He thought it was all ridiculous, this idea of being a solid citizen. And he went further and further off in the direction of being an outsider, mainly, in simple terms, of alcoholism. My mother was the opposite. Very together, figuring out how to get along."

Sam Shepard, right, and Michael C Hall in this year's Cold in July. Shepard says the trick to balancing the demands of writing, acting, theatre, film and novels is simply 'that I don’t do them all at once'. Photograph: c.IFC Films/Everett/Rex

I wonder how Shepard has for so long negotiated a career that has required careful calibration of his own outsider and insider inclinations, the placating of studio executives, producers, publishers, theatres, who see films, plays and books principally in commercial terms. "I don't get along," Shepard says gruffly. "It's difficult. I know as an actor you have to negotiate but I can't handle the whole idea that art and commerce are synonymous. It drives me nuts. And then you get the reputation of being difficult to work with." There are, he adds, producers who "seem to really care" but he's sceptical of "the big studio guys" and Netflix, and those people who "don't think what the actor's going through, what the writer's going through, what the artistic essence is".

Still, he enjoys acting, "now that I've figured out how to do it. I didn't like it at first. I didn't know where to look because this thing – the camera – was looking at me the whole time."

A theatre audience has always felt different, he says. And it is still theatre that lights him up. When he talks about it today – the physical space, the actors, the language – there is a reverence for it, a wonder. "You know, writing for the theatre is so different to writing for anything else. Because what you write is eventually going to be spoken. That's why I think so many really powerful novelists can't write a play – because they don't understand that it's spoken, that it hits the air. They don't get that."

He pauses, returns to thinking about his novel. "But of course I have the opposite problem," he concedes. "I can hear language, I can hear it spoken out loud. But when it comes into the head I have a much harder time."

Prose demands a carefulness he finds troublesome. "You need to be a lot more pedantic," he says. "I think. I don't know. The language of dreams is so different to the language of academics. It's beyond me. But the difference of spoken language and the language of the head is vast. Huge. It used to be all spoken. All out front and in the air."

These days, he reads a lot of Irish writers. "They are head and shoulders above," he says. "It's the ability to take language and spin it." And a lot of South Americans, too, "because they seem to have a handle on the ability to cross time and depth." He struggles to think of contemporary American writers he rates, beyond Denis Johnson. "The thing about American writers is that as a group they get stuck in the same idea: that we're a continent and the world falls away after us. And it's just nonsense."

Did he ever get stuck in that idea? "I couldn't see beyond the motel room and the desert and highway," he says slowly, and turns his glass a little. "I couldn't see that there was another world. To me, the whole world was encompassed in that. I thought that was the only world that mattered.

"And it's still there," he adds, "but now it's redundant because everything's replaced by strip malls."

The situation, he believes, is irredeemable. "We're on our way out," he says of America. "Anybody that doesn't realise that is looking like it's Christmas or something. We're on our way out, as a culture. America doesn't make anything anymore! The Chinese make it! Detroit's a great example. All of those cities that used to be something. If you go to a truck stop in Sallisaw, Oklahoma, you'll probably see the face of America. How desperate we are. Really desperate. Just raw."

But why, I ask, is the world still so infatuated with American culture? Why, even, do we remain gripped by a play such as True West? "Oh, because they all believe the American fable," he says. "That you can make it here. But you don't make it."

You've made it pretty well, I say.

"Yeah but I've also… I've… yeah," he hesitates, laughs, a long, rich wheeze. "But you know, oddly, I wasn't even fucking trying."

True West runs until 4 Oct at the Tricycle theatre, London NW6