Though best known to the American public as Chuck Yeager, Sam Shepard has spent most of his time for the past twenty years not as an astronaut, nor as an actor, but as a playwright. And it was as a playwright that Shepard came to Boston last month to stage a production on the Loeb mainstage with Shepard's friend and collaborator avante garde director Joseph Chaikin.

But Shepard announced to a group of Harvard students yesterday that he had decided to postpone the completion of that project with Chaikin. The production, originally slated for completion sometime this month, was funded by the American Repertoiry Theater (A.R.T.) but would have appeared on the Loeb Experimental stage, Jonathan E. Marks, University liason officer with the A.R.T., said yesterday.

Shepard and Chaikin have worked together on two such "performance pieces" in which the performance comes more from "improvisation than from the text," Marks said.

"We write together, play [music] together, move together," Shepard said. "Everything's open. We bring other people in...musicians, mostly."

Shepard talked to a group of sixty Harvard students about writing plays. Shephard, who does not like to keep a high public profile, wanted to talk with playwrights at Harvard, but only about a dozen people in the crowd wrote plays, according to Marks. The somewhat sottspoken midwestemer fielded questions from Harvard actors, directors and writers with utmost ease. As he leans back in his chair, taking a long drag on his cigarette, Shepard keeps the group of students captivate for over two hours by answering a wide range of questions.

When asked why he chose to be a playwright, Shepard answers that he did not choose it. He started out as a musician in a rock band, Shepard explains, and stopped playing music when it became more of a business and less of an aristic enterprise. Shepard describes his own career life as if it, too, were an improvisational play. "I don't see it as any kind of evolution. They [his plays] are all experiments in the dark."

"I'm interested in covering a lot of terrain. I'd like to design a city park, for instance," Shepard says, evoking a loud laugh. "I guess because I've been writing for twenty years I'm an writer. But I'd like to do a lot of things."

As an actor, Shepard has appeared once in a work of his own and a few times on film. He has misgivings about both: he describes his experience acting in his own work as "like being in an aquarium."

Though he has acted in movies before (such as portraying the farmer in the poignant but relatively unknown film Days of Heaven, Shepard came into the public eye when he played Chuck Yeager in the film rendition of the Tom Wolfe novel, The Right Stuff. Shepard explains why he accepted the role despite his disillusionment with Hollywood: his father was a pilot in the airforce, and as a friend of director Philip Kaufman, he decided that he wanted to take the part.

Though critical not only of the system of production of films but of the film medium itself, Shepard admitts that he would consider filming one of his plays, "Buried Child." Movies are "a kind of a ruse--an art ruse," Shepard says. "I don't feel like they have any real power to get inside, like theatre."

Though many of the questioners tried to pin Shepard down to descriptions of his own work, Shepard--as he has done in the past-refuses to be categorized. "I've got nothing to say," says the humble Shepard. He explains that the motivation for his work is not to convey any specific message, but varies from play to play.

Shepard deflects attempts by the audience to intellectualize his writing and directing. "You can't categorize these things," Shepard reiterates. "Life is too interesting to make it fixed."

Shepard fends off such questions as "Do you believe in God?" ("What God?") and "How do you approach Shakespeare?" ("I don't approach him.") For Shepard--who never attended college--the role of expert dramatist is still a novelty. "Now, to find it's inverted like that is shocking," he says.

But he gave a little concrete advice to aspiring playwrights. "You have to sit down and write...I mean, Nabokov wrote standing up," Shepard says, eliciting a big laugh.

Though most of his plays in the past have dealt with the theme of masculinity and relationships between males, Shepard explains that he is beginning to find that male-female relationships can be just as mysterious.

"This thing between men and women is starting to interest me."

No one asked his relationship with Jessica Lange-who recently gave a talk at Harvard--with whom Shepard is reportedly living.