SOMEWHERE BETWEEN the East and West Coasts, at least part of "Simpatico," one of the season's most anticipated plays, was written in a moving truck by its driver. That's Sam Shepard, who says he believes "all good writing comes out of aloneness." And you're not too likely to be interrupted driving along an Interstate.

"You have to do it on an open highway," explains the playwright during a lunch break for rehearsals of the drama, which opens tonight at the Joseph Papp Public Theater, with Mr. Shepard directing. "You wouldn't want to do it in New York City. But on Highway 40 West or some of those big open highways, you can hold the wheel with one hand and write with the other.

"It's a good discipline," he continues, "because sometimes you can only write two or three words at a time before you have to look back at the road, so those three words have to count. The problem is whether you can read the damn thing by the time you reach your destination."

Mr. Shepard delivers this bit of instruction in wary installments, smiling, it seems, at the absurdity of the image. A man heralded as the most original theatrical voice of his generation, he has traditionally regarded interviews with disdain and loathing. And he probably knows that once these words are in print, they will add further fuel to the myth of the playwright as an existential cowboy, forging art out of the wide open spaces of the American Plains. It is a myth, he says, he has never consciously tried to perpetuate.

"I didn't go out of my way to invent any image," says the 50-year-old Mr. Shepard. His lean, angular face, known to millions from his roles as an actor in movies, looks well lived-in now, more evocative of a Walker Evans photograph than of Gary Cooper. "I think those things are accrued as you go along, having to do with your work and certain things that have happened. If you set about trying to create an image, it's possible, I guess. But what do you come up with? Elvis Presley? And who's in charge?"

What he describes as "the cult of personality and the cult of image" disturbs Mr. Shepard. "I have no faith at all in that," he says. "I don't see that it has any substance."

Even in its most pedestrian sense, personality is something Mr. Shepard refuses to see as fixed. Throughout his 30 years as a playwright, he has created a series of fluid portraits of people for whom identity is, at best, a tentative proposition. The young man in "Buried Child" (which won the Pulitzer Prize in 1979) looks into the rear-view mirror of his car and sees his face dissolving into those of his ancestors. Hoss, the aging rock star of "The Tooth of Crime" (1972), complains of being "pulled and pushed from one image to another. Nothin' takes a solid form." And Henry Hackamore, the Howard-Hughes-like recluse of Mr. Shepard's "Seduced" (1978), has invented himself out of existence. "I'm dead to the world, but I've never been born," he chants.

IT SHOULDN'T COME AS A SURPRISE when a character in "Simpatico" asks, "How many lives do you think a man can live?" It is in some ways his most conventional work, in its structure and emphasis on naturalistic dialogue over the poetic arias that became Mr. Shepard's dramatic signature. But his obsession with mutability still shines through.

The play is in a sense a reworking of themes in "True West" (1980), in which two brothers, a proper screenwriter and a thieving vagabond, gradually exchange identities. "Simpatico" follows the consequences of the framing of a California racing commissioner by two men (played by Ed Harris and Fred Ward) years after the incident. Mr. Shepard estimates that he has written eight or nine unfinished plays around the two characters, Vinnie and Carter. "It's an old, old situation that I've been struggling with for years," he says.

The patterns of that situation were clear even in the chronologically scrambled sequence of scenes in a long day of early rehearsals. As the characters, who also include Vinnie's mistress manquee (Marcia Gay Harden), the wife the two men shared (Beverly D'Angelo) and Ames, the man they done wrong (James Gammon), faced off, always in combinations of twos, the very foundations of self seemed to quake.

"Identity is a question for everybody in the play," says Mr. Shepard. "Some of them are more firmly aligned with who they are, or who they think they are. To me, a strong sense of self isn't believing in a lot. Some people might define it that way, saying, 'He has a very strong sense of himself.' But it's a complete lie."

The same sensibility infuses Mr. Shepard's views on the relations between men and women, which in "Simpatico" are scarcely hopeful. At the time of his play "Fool for Love" (1982), which portrayed an incestuous affair as a knockabout fight, Mr. Shepard described what happens between the sexes as "terrible and impossible." Asked if he still believes this, he takes his time answering.

"The whole thing between men and women is really the most amazing thing" -- pause -- "there is," he says. "But, yeah, it's impossible the way people enter into it feeling they're going to be saved by the other one. And it seems like many, many times that quicksand happens in a relationship when you feel that somehow you can be saved. And of course that's going to be a disappointment." He laughs fully. "In that sense, yeah, I think the illusions about it are impossible." Does he still have those illusions? "No," he says, "I don't."

The fantasy of disappearing from the known world is woven throughout Mr. Shepard's plays, and he has been pretty good at it in real life, in spite of his status as a movie star. "I think most writers, in a sense, have this desire to disappear," he says, "to be absolutely anonymous, to be removed in some way: that comes out of the need to be a writer."

For a while, it looked as if Mr. Shepard had disappeared from the New York theater. Following a brilliant cycle of family-themed plays -- which included "Curse of the Starving Class" and "Buried Child" and climaxed with the searing, ambitious "A Lie of the Mind" in 1985 -- there were seven long years of absence, during which Mr. Shepard appeared as an actor in movies (including such froth as "Baby Boom") and directed one, "Far North (1988). (A second, "Silent Tongue," was released early this year.) When in 1991 he returned to the New York stage with "States of Shock," an allegorical play about a Vietnam veteran and his fascistic military keeper (John Malkovich), he was speared by critics. There was speculation that the playwright's skills had been left fallow for too long.

"I think there was an intrinsic misunderstanding about it," he says, "which was probably my fault. I think the audience, and obviously everybody else, had a hard time realizing that this was indeed about a father and son relationship."

Theater, he adds simply, is his natural element. "For one thing, it allows you to explore language, which film doesn't. Film is anti-language. I don't know why that is, but that's the way it developed. The other thing is the relationship between actor and audience: that moment-by-moment hanging in the balance, that terror of the moment.

"Theater combines everything, for me, anyway," he says. "It's like you pick up a saxophone and you play a saxophone and that's it. It's a partnership. I feel at home with it."

"SCAT!" YELLS MR. SHEPARD, from the shadows at the back of the Newman Theater at the Public. On stage, Marcia Gay Harden, who has been holding her ground behind a sofa as Mr. Harris advances menacingly toward her, looks up like someone wakened from a dream.

"Scat!" Mr. Shepard continues, with comic urgency. "Git out of there! Scat!"

Ms. Harden starts to move backward, hurriedly. Mr. Harris follows, bumping into furniture. The scene has developed a fever; it now seems to sweat as well as pulse.

"Heh, heh, heh!" The playwright's laughter lances the darkness like an illuminated smile. It's the first day that Mr. Shepard and his company, after four weeks in a rehearsal room, have moved into the theater. Loping silently, in jeans and moccasins, on his rangy stork legs, arms akimbo, Mr. Shepard cuts a sharp-edged silhouette against the glow from the stage. As he watches his words assume different lives with each repetition, he does indeed seem very much at home.

Mr. Shepard experimented a little with live acting when he lived in New York in the 1960's and early 70's. But with his aversion to "confronting large groups of people," he didn't do much. "Too spooky," he says.

Directing "Simpatico," he is respectful, affectionate, almost courtly, usually prefacing interruptions with apologies: "I'm sorry to stop you" or "Excuse me."

The previous weeks had been, he says, a period of letting the actors get to know their characters with little directorial intervention. "It's a funny thing about freedom with actors," he says. "You invite them into certain scary territory; then it becomes a question of how far you let them go into that territory before you start shaping it. I'm a firm believer that so-called blocking doesn't come out of the director. If the actor has any kind of chops at all, he's going to find his way around the stage and find the impulses. To order actors around the stage like a general is not my idea of a director."

On this day, he seems particularly intent on cadence. "All the unspoken structures of playwriting are very close to music," he says. He often tells his actors to "hit the notes," though other forms of imagery also come in handy. As Mr. Ward's Vinnie lies on a rumpled bed in a hungover depression, Mr. Shepard tells him to imagine a buzzard circling overhead. To Mr. Harris, whose character undergoes the play's most devastating transformation (into what the actor describes as "a quivering ectoplasm"), he advises, "I think what we're shooting for are the signs of the earthquake."

Mr. Harris, whom Mr. Shepard last directed in "Fool for Love" 12 years earlier, finds the playwright "more comfortable, easier" than before. "You never get any feeling of panic," he says. When necessary, the director can be very firm. "You know," says Mr. Harris, "my character is full of guilt and lies. And I was asking Sam, isn't there some place where he admits to all that and gets close to it? He said, 'Sorry Ed, I can't let you off the hook.' "

Much of Mr. Shepard's guidance seems built, as Mr. Harris puts it, around "his sense of the aloneness of these people." Many of the director's blocking suggestions put greater distance between the actors. He also admires the darkness of the scrim behind the sets, a consequence of a blown-out fuse, because the black space underscores a sense of isolation. He quickly shifts into parody. "Very existential," he hoots with a French accent, striking a hieroglyphic pose. "Very Jean-Paul Sartre."

THERE WAS A TIME WHEN MR. Shepard might have spoken those words in all earnestness, when he came to New York in 1963. A restive, Beckett-reading 19-year-old Californian with three semesters at a junior college and a stint as an actor with a church-touring amateur troupe behind him, he soon fell in with the experimental, European-flavored theater groups blossoming on the fringes of Off Broadway.

His avowed ambition was to become a rock star ("The last thing I'd want to be now," he says). And Mr. Shepard, who has previously spoken about his wide-ranging experimentation with drugs in that period, describes himself in his 20's as "crazy and mixed-up and confused like everyone else: sort of raw, emotionally raw." He adds, using a phrase that shows up in "Simpatico," "I was fishing in the dark." Isn't he still? "Yeah. But not like that. Not to the point where you're a candidate for self-destruction."

He was also spinning out plays with the speed and facility that youth allows, though he says the story that he wrote his first produced play, "Cowboys" (1964), on the back of Tootsie Roll wrappers is nonsense. From the beginning, his penchant for electric, inventive language and his preoccupation with the myth of the vanishing West were in evidence, in works that often took the form of sustained hallucinations.

Asked if his 1971 play "Cowboy Mouth," which he wrote while holed up with the cult rock star Patti Smith at the Chelsea Hotel, wasn't an immediate translation of a collaboration, he answers with an explosive laugh: "It was an immediate translation of a crisis. Yeah, that's one of those plays that just kind of spewed itself out there. No craftsmanship at all. Just pure emotion."

In the succeeding years, the traditions of playwriting have become more important to him. "Who was it who decided to do away with all the plots?" asks a character in "Simpatico." It is, in fact, by far Mr. Shepard's most densely plotted work.

Today, he writes slowly and carefully, he says. "One of the things that's become apparent to me over a long time is that no matter how you cut it, plays are about storytelling. You know, in the 60's, everybody was down on it. It became an old-fashioned, archaic structure. There was a huge breakaway with those European writers like Beckett and Ionesco and Arabel." Now, he says, "I think you need to include all these notions that at one time you rejected as being part of the established order of things. There's no reason, uh, to shoot yourself in the foot."

From early on, in plays like "The Tooth of Crime" and "Geography of a Horse Dreamer," he seemed to be anticipating the problems of the success that would overtake him. He soon developed "agoraphobia," which he translates to mean "fear of the marketplace."

"Everything is co-opted in this culture," he says. "No matter what happens, there's always this marketing aspect that seems to leap on top of everything like a vampire." It is a process he knows well from his two efforts as a film director, of which he says, "The problem there is you don't have the luxury of learning from your own mistakes. You just get the ax, and that's it."

The same process has occurred with Mr. Shepard's plays, most memorably in his last previous experience with the Public, in 1980, when the producer Joseph Papp, against Mr. Shepard's wishes, took over the direction of "True West" from Robert Woodruff. "He told me he owned my play," Mr. Shepard said. "And he proceeded to do anything he wanted with it, as though it was a used car that I'd sold him." He never spoke again to Mr. Papp, who died in 1991.

Mr. Shepard's escalating fame as a movie star, which began after his appearance in Terrence Malick's "Days of Heaven" in 1978, presented other problems, exacerbated by his breathlessly chronicled relationship with Ms. Lange, for whom he left his wife, the actress O-Lan. "I still haven't gotten over this thing of walking down the street and somebody recognizing you because you've been in a movie," he says. "There's this illusion that movie stars only exist in the movies. And to see one live is like seeing a leopard let out of the zoo."

He has also been appalled by "the banditry" of the media. "I came to realize that in many cases, you're being duped," he says of his experiences with interviewers. "You're being led to believe that what they're interested in is what you have to say. But what they're really interested in is the manipulation of your personality for the sake of selling it to a magazine." After the publicity around his film "Far North," in which Ms. Lange starred, he decided to call a near moratorium on interviews. "It was like I'd volunteered to go to my own hanging," he says. "Thanks a lot."

Mr. Shepard's sustained public silence has fostered a perception of him as a Garbo-like sphinx, a notion that makes his friend and longtime collaborator Joseph Chaikin, the founder of the Open Theater, roar with laughter. "He's really very easy to talk to," says Mr. Chaikin.

Indeed, for anyone at all familiar with Mr. Shepard's biography, it is clear that his most intimate experiences and feelings gleam piercingly throughout his work. In fact, his work may be the most intensely personal of that of any living playwright of his stature, which may account for the fact that his style (unlike, say, David Mamet's) has never been successfully imitated. In a reading given by Mr. Shepard in early October at the 92d Street Y, the self-portrait that emerged from his selections from his plays and autobiographical writings seemed astonishingly naked. It was almost embarrassing to meet him later.

"Simpatico," on the other hand, would seem to be less directly rooted in Mr. Shepard's life than many previous works. "It seems that way," he says pointedly, laughing. Certainly, the playwright's familiar theme of father and son -- for which Mr. Shepard has admitted he drew from his complicated relationship with his own father, a salty, hard-drinking man of military discipline -- would appear to be absent in "Simpatico."

Mr. Shepard isn't so sure. "The odd thing to me is I think all of those relationships are inside other relationships," he says. "Two friends can have a father-son relationship or a brother relationship. Those things aren't necessarily expressed by external character. There are these territories inside all of us, like a child or a father or the whole man, and that's what interests me more than anything: where those territories lie.

"I mean, you have these assumptions about somebody and all of a sudden this other thing appears. Where is that coming from?

"That's the mystery," he says, looking for a moment very young. "That's what's so fascinating."