Sam Shepard at the Citizens

True West, an extended snapshot of what can happen to estranged brothers who spend too much time together, is an unsettling play that has clear themes, dramatic tension and flashes of broad humour, even slapstick, but little narrative arc or plot resolution.

So the characters live on in your head, long after the actors have left the stage, like the unresolved itch of something you intended to do.

But tonight it could be different. Tonight we might get the full story from the man himself - playwright, musician and Hollywood actor, Sam Shepard.

Most of the audience at the Citizen's Theatre stay on for his question and answer session after the play, but a few head home, leaving vacant seats in the stalls. So with the usher's permission we abseil down from the upper circle, in time to see the rangy 70-year-old amble onstage in blue jeans and dark coat, and take a seat beside the play's director Phillip Breen. 

Shepard has the wary look of someone facing a Glasgow audience for the first time, and his initial answers are monosyllabic. A question about writing clearly relaxes him and he starts to open up.

"You don't necessarily have a lot of motivation or predestination about where you're going," he says. "You just start. Characters, situations and places appear, but there is never a lot of thinking about it. That takes place as you're writing."

Vivid images of the origins of the play we've just watched remain in his mind, three decades later, he says. "The sound of crickets and coyotes, the presence of the desert. Los Angeles is the weirdest place in the world. 

"Why do we plant a city in the middle of the desert like that? It's like gobbled up by demons."

A powerful presence in True West, the desert is where one brother lives and the other yearns to go, hoping its demons will restore a grip on reality weakened by years of writing for a living. 

It's a play born out of Shepard's own struggles with the script of what turned out to be a cinematic masterpiece, Breen explains on the Citizens website. "One can just hear the conversations with potential producers of Paris, Texas and Shepard's own self doubt in some of True West's most blistering dialogue: "'In this business we make movies. American Movies. Leave the films to the French.'"

Shepard is less analytic about the products of his creative efforts, but chats fluently about the process. Asked which brother in the play is based on himself, he says "Everything you write is based on yourself, no matter how much writers want to deny it. I've read over and over that Shakespeare is the one playwright who stays out of his material.

"Bullshit. Every character that Shakespeare wrote is himself. Yes?"

An appealing feature of his interactions with the audience is this element of dialogue. He poses questions as well as answering them. "Does that make sense?" "Do you know the man?" "Have you been to Minnesotta?"

Shepard ranges over writing and acting, his early days in Hollywood and his thoughts on Meryl Streep, with whom he recently acted. "She comes in and it's like a leopard let loose in the room. You have no idea what's going to happen next."

He even chats about Patti Smith, his former lover and still a friend, he says, whom he recently accompanied on the banjo, when she recorded Smells Like Teen Spirit.

With the thoughtful, engaging air of a guest at your dinner table, Shepard makes the effort to answer every question put to him. But there are aspects of any work of the imagination that can't be pinned down, even by the one person you might expect to know. 

As the session draws to a close, a woman in the audience asks what happens to the two brothers we have just spent an evening with, after the curtain has fallen on True West.

Sam Shepard shakes his head. "I have no idea," he says.