SAM SHEPARD is a hard man to find.

It is said that he lives in Mill Valley, Calif., in the heart of hip Marin County north of the Golden Gate. But it also is said that he likes to change his telephone number frequently, and even his friends often have to wait for him to call before they can reach him. Don't plan to drop in unexpectedly, either, even if you've been assured he's at home.

The visionary playwright and fledgling movie star, whoser "Curse of the Staring Class" opens at Arena Stage's Kreeger Theater Wednesday, was rumored to be at one of two Marin addresses last weekend. The first, called the Flying Y Ranch, appeared at the end of a crunching drive down a muddy country path which overlooks spectacular vistas of the Marin greenery and the bay. A woman looked up from grooming her horse and said Shepard had moved away two year ago.

The other address is a little hacienda on a quiet residential street. A white Chevy Nova was in the direway last weekend. Compared to the Flying Y, this place seems a little tame. But the man across the street confirmed that Shepard lived there. And through the front door window could be seen the cover of Shepard's "Rolling Thunder Logbook," mounted on the wall inside.

Not so fast, said the younger of two women who answered the door. Sam Shepard wasn't there. In fact, she didn't know Shepard. She had never met him. Why display his book cover on her wall? She was a fan, she said.

Good-bye, she said.

Several hours later, a man answered the same door. The name was John Dark, and he showed his driver's license to prove it. No, Shepard didn't live here. The "Rolling Thunder Logbook"? He had bought it in a bookstore downtown. Say, he suggested, maybe the owner of the bookstore could locate Shepard.

The bookstore owner could not be found. But the "Rolling Thunder Logbook" could, and the first chapter begins like this: "Johnny Dark is behind the wheel. The white Chevy Nova is rolling down through San Anselmo... " Shepard goes on to describe how Dark was chatting with him about Bob Dyland on the same day Dyland called and asked Shepard to join Dylan's Rolling Thunder Revue.

What was going on here?

Sam Shepard, theatrical navigator of American myths, is becoming something of a mythic figure himself in the American theatrical world. As his fame and influence increase, he is becoming more introspective and private. He is a man of mystery, and this probably pleases him no end.

For years coteries in New York, London and San Francisco have been crazy about him and his plays, which now number around 30. Critic Martin Gottfried has called him "the pure artist, the prophet of technological mysticism, surely the most influential young playwright in America." Shepard has won seven of off-Broadway's Obie awards, including a controversial prize for "Curse of the Starving Class" as "Best Play/Best New American Play" before it had even been given an American producation.

Shepard has better countercultural credentials than any other American playwright. He has worked with Dylan and Antonioni. He has been close to Patti Smith, who worked on a script with him and wrote a poem about him. He is married to an actress named O-LAN(they have one son). He lives in Mill Valley.

Now he is reaching a wider audience. Arena gave a workshop production of his "Angel City" last season, and Arena's "Curse" will be Shepard's full-scale introduction to Washington. Even more important in terms fo pulbic recognition, Shepard is becoming a movie star.

He played the isolated, lovestruck farmer in "Days of Heaven," last year's cinematic objet d'art. And next week he bgins rehearshing a role in "Resurrection" for Universal Pictures, costarring with Ellen Burstyn and Eva Le Gallienne.

Despite all this -- or perhaps because of it -- Shepard is turning inward. His friends have been well trained to protect his privacy, and one of his current interests is the know-yourself teachings of the philosopher Georgi Gurdjieff.

His recent plays reflect his impulse for self-examination. Much of his earlier work was so surrealistic that it was difficult to pinpoint traces of Shepard's actual experiences in it. But "Curse of the Starving Class" is set on actual experiences in it. But "Curse of the Starving Class" is set on a California avocado farm, and Shepard spent most of his teen years on an avocado farm near Duarte, Calif. The more recent "Buried Child," now playing in New York, is set in a small town in Illinois. Shepard was born in Fort Jefferson, Ill., in November 1943.

In both plays, crazy families seem on the verge of collapse, yet they ferociously remain together by clinging to American family myths. Shepard's style is much less surrealistic than before, and it appears that he is plumbing his own past for his drama.

He begam life as a transient. His father's army service took the family to such outpost as Rapid City, S.D., and Guam. Finally they settled near an aunt in Pasadena who had some moey. Their home was surrounded by 65 avocado trees and a few sheep. Theatergoers will note that a lamb is one of the supporting players in "Curse of the Starving Class," and Shepard has said he coveted a county fair prize for his ram. Shepard was something of a shepherd.

School apparently didn't interest young Sam much, but he was fascinated by some of the characters he met there. One of them, an Elvis clone, inspired the character of the fading rock star in "The Tooth of Crime," which was Shepard's best known work prior to "Curse."

Shepard entered the theatrical world via the Bishop's Company Repertory Players, a traveling troupe devoted to such mild fare as Christopher Fry plays and "Winnie the Pooh." In 1963 Shepard left the troupe and became came a struggling New York actor, but within a couple of years he was struggling much more successfully as a playwright.

His own plays were not mild. The last scene of "The Rock Garden" -- one of two one-acts that in Cotober 1964 were his first produced plays -- was later incorporated into "Oh, Calcutta!"

Shepard's next effort was a triple bill of one-acts at La Mama Theater Club in February 1965, and Shepard has recalled them with remarkable objectivity: "Up to Thursday' was a bad exercise in absurdity, I guess. Thid kid is sleeping in an American flag, he's only wearing a jockstrap or something, and there're four people on stage who keep shifting their legs, and talking.... It was a terrible play, really."

"Dog" and "Rocking Chair" were on the same bill. The former was "about a black guy -- which later I found out it was uncool for a white to write about," Shepard has said. "I don't even remember 'Rocking Chair,' except it was about somebody in a rocking chair."

"Chicago, his next play, was published in a 1966 anthology of eight off-off Broadway plays, and it introduced Shepard to London in 1697. Eventually nearly all of the off-off Broadway citadels of the '60s -- including Theatre Genesis, Caffe Cino, Jusdon Poets' Theater and the Performance Group -- produced Shepard's works. So did more established institutions. The American Place Theatre introudced his first full-length play, "La Turista," in 1967, and the Repertory Theater of Lincoln Center presented his "Operation Sidewinder" in 1970.

In the early '70s Shepard lived in London, and most of his work of the period was presented there first, including "The Tooth of Crime." The BBC presented his "Blue Bitch."

But even in London Shepard's work was filled with images of the American West, and since 1974 Shepard has worked out of San Francisco, where he is palywright in residence at the Magic Theater.

Among the plays introduced at the Magic was "Angel City" in 1976, the first of Shepard's clearly autobiotions Hollywood sets before young writers in scathingly satirical terms. Apparently Hollywood didn't mind the joke; Shepard the actor is finally becoming known through Hollywood films.

He has not, however, forgotten his initial theatrical desire to appear on the stage. Last June the Magic presented him and experimental theater doyen Joseph Chaikin in a collaboration called "Tongues," which required Shepard to play a variety of percussion instruments.

Shepard, like his father, has been a drummer for a long time, and in the late '60s he played in a group called the Holy Modal Rounders. His writing is often described as musical rather than dramatic, with long fantasies interrupting the dramatic flow in the manner of improvisations. Recently several critics, including Walter Kerr and John Lahr, have chided Shepard for a lack of dramatic cohesion.

But Shepard's muscial interst must hav impressed Bob Dyland, who hired him to write a screenplay about his Rolling Thunder Revue as it toured America in 1975. The film never materialized, but Shepard's "Rolling Thunder Logbook" was published in 1977.

In an article in The Drama Review a year ago, Shepard defended his interest in myth and his indifference to ideas. The adherents of a theater of "ideas," he wrote, are "almst always referring to dieas which speak only to the mind and leave out completely the body, the emotions, and all the rest of it. Myth speaks to evertyhing at once, especially the emotions. By myth I mean a sense of mystery and not necessarily a traditional formula. A character for me is a composite of different mysteries. He's an unknown quantity. If he wasn't, it would be like coloring in the numbered spaces." Shepard's new play "Seduced," which opens in New York soon following a Providence premiere, tackles the subject of Howard Hughes, who personified this definition of myth better than anyone.

Shepard also expounded on the musical method of his writing: "From time to time I've practiced Jack Kerouac's discovery of jazz-sketching with words. Following the exact same principles as a musician does when he's jamming. After periods of this kind of practice, I begin to get the haunting sense that something in me writes but it's not necessarily me... This identical experience happened to me once when I was playing drums with the Holy Modal Rounders, and it scared the s -- out of me. Peter Stampfel, the fiddle player, explained it as being visited by the Holy Ghost, and it seemed reasonable enough at the time."

If the Holy Ghost is planning on visiting Sam Shepard at home, good luck. Like Hughes, Shepard is trying hard to remain a composite of different mysteries, an unknown quantity, and divine powers may be required to ferret him out when he's not in a talking mood.

But he's out there somewhere. Last week the phone rang in the Marin County tax assessor's office. Could the clerk please look up the name of the owner of a little hacienda on what shall be called Meloow Lane, where the Chevy Nova was parked last weekend?

Sure, said the clerk. A few papers could be heard shuffling, and back came the answer: "Sam Shepard and O-Lan Shepard."

Even myths must pay their taxes.