In a bright room in Derry, in the mid-afternoon, two men meet and immediately start speaking in riddles.

“Your idea of the miniature was good.”

“Yeah, I just don’t know how to get around the hugging thing.”

“Is it possible that Stephen could embrace the shadow, or no?”

“I can’t tell, because I’m not sure what the shadow is projecting on to.”

“A wall, hopefully.”

“Yeah, but if the shadow is 15ft high, what’s he hugging?”

“You have to make him as big.”

“It’s tough to get him as big as the shadow unless they’re the same size.”

“So we need a miniature Stephen.”

The first man is Sam Shepard, easily the most acclaimed American playwright of his generation (he turned 70 earlier this month). The second is Sean McArdle, who, as Shepard's prop master for eight years, has been helping to solve some of the challenges of his theatre: an exploding ceiling fan in 2009's Ages of the Moon, for instance, or a dead horse in 2008's revival of Kicking a Dead Horse, both for the Abbey.

Here, the two men are considering the death scene of Jocasta, mother and wife to Oedipus, in Shepard's fragmented new version of the myth, A Particle of Dread (Oedipus Variations), which is being premiered by Stephen Rea's Derry-based company, Field Day. The conversation between the writer and the prop maker is no-nonsense and good-humoured, but, to the uninitiated, it makes even the riddle of the sphinx seem straightforward.

It also begs a similar question. If the morning of Shepard’s career was defined by experimentation, guided by the fracturing culture of 1960s America and his jagged imagination, then moved into a noon of naturalism, tragedy and exploded American myths, what did an evening return to one of the crutches of western theatre suggest? As Oedipus himself wonders, but realises too late, what is he looking for?

Shepard was first introduced to Oedipus Rex by his frequent collaborator, the late director Joseph Chaikin, in the early 1960s at New York's Open Theatre, where Chaikin used the play's "recognition scene" as a tool for developing actors. "I was too young and arrogant to even dip into it," Shepard admits today. "I thought it was over my head, actually."

Some years ago, he attempted a straight adaptation (“it’s just straightforward really,” he realised, “it’s about murder”) but he gave up on the idea when he felt there was nothing to add to Sophocles. At Rea’s invitation he returned to it, workshopping a fragmented version whose scenes dart through place and time, and that relies heavily on live music.

"That's why he calls it Oedipus Variations," says the show's director, Nancy Meckler, also a longtime associate of Shepard's. "It's not a 'well-made play'. Sometimes we're seeing what might have been speeches from an antique text, other times modern characters whose lives seem to be touched strangely by the Oedipus story."

As any psychiatrist will tell you, everybody’s life is touched by the Oedipus story. Freud famously found in Oedipus a sexual archetype blown up into extreme proportions: the man who unwittingly kills his father and marries his mother. “I don’t think that interpretation interests Sam,” says Meckler. (“I’m not interested in the psychological aspect,” Shepard says as soon as the idea is trailed.) Instead of the Oedipal complex, Shepard frequently refers to Oedipus’s predicament. “The predicament,” as Meckler puts it, “of not knowing your origins.”

Just as Shepard's theatre is frequently engaged in the impossible search for authenticity, the deep roots that might make sense of it all, Oedipus Rex is often called the first detective story.

The city of Thebes is cursed with a plague and its ruler, Oedipus the King, must find the killer of the previous king in order to lift it. He looks for leads, follows clues, blinded to the fact that he is the target of his own manhunt. Shepard’s play takes its title from a line spoken by the Chorus in a 1950s translation of Sophocles (“If the killer can feel a particle of dread, Your curse will bring him out of hiding”) and one scene seizes on the detective genre explicitly. “They all tell a story,” says a forensics expert as he inspects clues at the scene of a brutal roadside murder. “The story of what happened. The past.”

Attractively parched
Sitting with a cup of tea in the bar of Derry Playhouse, his voice is attractively parched, hair brushed back as though standing upright in shock. You might expect Shepard to be elusive with the details. He rarely does interviews and it's surprising to hear that the night before we meet he took part in his first ever Q&A session for a production of his seminal play True West in Glasgow's Citizen's Theatre. ("It steered away from meaning, thank God," he exhales.)

In person, however, he is genial, quick to laugh and warmly forthcoming.

“It’s the difference between reason and instinct, if you want to be intellectual about it,” Shepard says of the detective scene. “That’s what I was trying to set up. But the thing about Oedipus to me that is so incredible is that it doesn’t have a plot. There’s no story. It’s just a situation. It’s a predicament that the central character finds himself in. And the audience knows everything. He’s totally guilty, as the audience knows, but believes himself totally innocent.”

You could say that reason and instinct have been the twin forces through Shepard’s career as a writer, the contradiction in the “cowboy poet”.

“Oh, of course, yeah,” he says. “You’re continually asked questions having to do with the logic of something: ‘Why is this here rather than this? What does this mean?’ What’s never taken into consideration is that you don’t work that way as a writer.” He laughs. “The person asking that question can’t possibly know that you don’t know.”

In collaboration, then, Shepard prefers the company of instinctive artists. (“He’s a very non-psychological actor,” Shepard says of Rea, with earthy admiration. “He’s not a Lee Strasberg actor.”) It seems significant that Field Day’s production reunites him with Rea and Meckler, who first introduced the men in early 1970s London, and both Meckler and Rea describe working with Shepard in exactly the same terms. They compare it to jazz.

When Shepard was directing Rea in the Abbey's Kicking a Dead Horse, the actor would frequently depart from the script during rehearsals. "Oh, I was just kind of thinking of John Coltrane and how he did that improvisation around My Favourite Things," Rea might explain. "It stuck with me for a while," remembers Shepard. "It came back to me when I was working with this thing. Why not just do variations on Oedipus? Why not just take off?"

Classic tragedy
Shepard will readily concede any influence on his writing, but where people see the fingerprints of classic tragedy in Curse of the Starving Classes, say, and Fool for Love, or Pinter's whisper of menace in Geography of a Horse Dreamer, or the vaudevillian existentialism of Beckett in Kicking a Dead Horse, they are slower to see the presence of Elvis Presley, Marlon Brando or film noir. "Equally important!" says Shepard. "Absolutely. Beckett was incredible but so was Jerry Lee Lewis. So was Little Richard."

Beckett, though, has become an intriguing touchstone, and Shepard reaches for one anecdote of Rea's, whom Beckett himself directed in Endgame, when Rea asked whether his character, Clov, really disappeared into the kitchen. "Beckett said, 'It's always ambiguous.' I thought that was the most fantastic thing. Beckett also had an idea about the 'mythological present'. That was a tense that he used. Which is quite extraordinary. It makes you gasp. When he says, 'I am', he's talking about eternity."

The “mythological present” might be Shepard’s natural mode too. “I’m still trying to work out the difference between fate and destiny,” he says. I know that destiny is the thing that you’re written to do and fate is perhaps the thing you do with it, or vice versa. But the idea that, regardless of what you do, this thing has already come down; it is already written.”

However you vary the telling, Oedipus, and his eternal predicament, will always be with us.

A Particle of Dread (Oedipus Variations) is at the Playhouse Theatre, Derry, until Saturday. It is reviewed below