On the Folger Theatre's ceiling are printed some oft-quoted words of William Shakespeare's whose gist is this: Whoever we are, whatever we do, wherever we do it, it's theater.

Only a definition that generous would cover "Benefit of a Doubt," the work that opened at the Folger last night. In Edward Clinton's new play, ideas fly and die with the suddenness of Vanguard missiles, and the characters, only fitfully worthy of our belief, are scarcely ever worthy of our concern.

Clinton has concocted an Appalachian family plagued with problems beyond the Waltons' wooziest nightmares. Eileen, the mother, went crazy one night watching "The Merv Griffin Show," smashed a picture window and left for Pittsburgh. Her jokester of a coal-mining husband, John, wants desperately to leave the mines and open a hamburger franchise. He also has it in his head to commit his religion-crazed mother-in-law to a rest home.

They have a retarded 14 year-old daughter, too, who carries a doll named Bosco around with her everywhere and worries that the neighbors think she is strange. Her secret friendship with a gin-drinking black man named Dandelion, and her mother's return home (and subsequent unwanted pregnancy) provided most of what there is in the way of plot.

Nestled in among these eccentric characters and their tensions are some inspired fragments of dialogue and story-telling.

The husband pays a nostalgic visit to his old car, now at a used-car lot. Complaining to his wife about her coldness, he says: "We could be two pork chops lying in bed together." Their daughter, learning to deposit a pickle on a fast-food hamburger after a machine has squirted it with ketchup and mustard, proudly announces, "I guess I'm the human touch."

But the disjointed virtues of the play are, for the most part, manhandled by a crude job of directing and acting.

Director Barnet Kellman has cast the play with wrinkly, down-home faces and gravelly voices that would certainly be at home in coal country. But Stephen Mendillo as John and Geraldine Court as Eileen hardly ever vary those first, plausible impressions. What ought to be the key scenes between them are stiff, unsubtle and vastly unmoving.

As the daughter, Carol Kane shows much more spirit. While she has broken no new ground in the difficult business of playing a retarded character, her scenes-particularly with Elizabeth Council as her grandmother and Ray Aranha as her friend-come the closest of any in the play to being fully articulated and fully credible.

In the thick catalogue of poor choices made by his play's director and producer, one of the poorest is Philipp Jung's stylized, unconvincing set, with a realistic interior back-dropped by a preposterous primitive painting of urban Appalachia.

This confused set neatly parallels the confused purpose of the work performed on and in it.

Like more and more new plays, "Benefit of a Doubt" has been through a long haul of shaking down and rewriting, plus one staged reading in Rhode Island and one full production in Cincinnati. Unless the play's ultimate destination is the author's trunk, this version isn't ready yet. CAPTION: Picture, Carol Kane; by James A. Parcell