Magic Theatre; Sam Shepard drama assaults audience, actors - and scenery; Fool for Love Drama by Sam Shepard. Directed by Mr. Shepard.

The Magic Theatre of San Francisco has come to town with a production of a new Sam Shepard opus that makes extreme demands on its actors, audience, and scenery. In some respects, especially the scenery.

Andy Stacklin's deliberately hideous, green-painted motel-room setting features two doors, one on each side of the stage. The repeated door slamming not only creates a horrendous percussive effect but punctuates the hateful struggle that occupies Mr. Shepard's ''Fool for Love.'' It assaults the characters and assails the audience.

An acclaimed award-winning dramatist, Mr. Shepard can endow everyday vernacular with eloquence. He finds humor, however caustic, even in the midst of desperation. His harsh views are intelligently expressed. But somewhat in the manner of a Hitchcock, he employs his clever techniques to make the audience squirm.

At times, the physical demands the author-director imposes on his actors stir concern for their safety, particularly as they slam one another into the walls of the cell-like set. It is as if Mr. Shepard were prepared to risk injuring them for the sake of theatrical effect.

The struggle on the edge of the Mojave Desert pits May (Kathy Baker) against Eddie (Ed Harris), the itinerant cowboy lover who has repeatedly abandoned her in the course of their 15-year relationship. A grizzled, ghostly Old Man (Will Marchetti) watches the battle from a side-stage rocking chair. His rambling reminiscences haunt ''Fool for Love,'' suggest a bigamous past, and intimate that May may be Eddie's half sister.

But as is often the case with a Shepard play, separating fact from fancy can be highly subjective and speculative.

What requires no speculation is the bitterness and betrayal that have led up to this climactic confrontation. The long ordeal has exhausted May, while the contentious Eddie has become virtually psychopathic, his articulateness an instrument of his cunning.

Because of its excesses, ''Fool for Love'' proves painful but not moving, lacerating but not redeeming, sensational but emotionally starved. The characters may cry but the spectator (at least this spectator) remains untouched. The work is badly wanting in tender mercies.

As Mrs. Fiske once said of Ibsen's ''Ghosts,'' Mr. Shepard has written a play in which ''the sins of the fathers are visited on the audience.'' And those sins include what Mr. Shepard regards as the degeneration and betrayal of the American dream.

''Fool for Love'' begins in wretchedness and ends in bleak uncertainty. Deserted once more by Eddie, May is left with her new boyfriend, Martin (Dennis Ludlow), the kindly, bewildered, inadvertent rival whom Eddie has taunted and bullied. Mr. Ludlow is the fourth member of the excellent San Francisco cast.

The production at the Circle Repertory Company was lighted by Kurt Landisman and costumed by Ardyss L. Golden.