"Who Am I This Time?" is a ripe slice of honeydew, a really beguiling charmer, and the fourth program in the much-needed "American Playhouse" series on PBS.

Morton Neal Miller wrote and produced this adaptation of a Kurt Vonnegut Jr. short story, to be seen at 9 tonight on Channel 26 and other public TV stations. The other personnel involved are unusually lustrous for a domestic public TV production; the director is Jonathan Demme, who made "Citizens Band" and "Melvin and Howard," and the stars are the much-talked-about Christopher Walken, who does the slinky dance in "Pennies from Heaven" and played Russian roulette in "The Deer Hunter," and Susan Sarandon, whose best work previously was the Louis Malle film "Atlantic City" and whose big imploring eyes bring to mind those of the late Natalie Wood.

Walken plays Harry Nash, the hero of the piece and the star performer of the North Crawford Mask and Wig Club, whose productions Harry regularly graces with extravagantly passionate performances. Then Harry slithers off and returns to his life's work, the back room at a hardware store. Sarandon, as Helene Shaw, arrives in town with the new billing machines at the phone company, and is recruited to play Stella to Harry's Stanley in the local production of "A Streetcar Named Desire."

Harry literally throws himself into roles -- his "Cyrano" had a stagehand so teary he forgot to ring down the final curtain -- and when he pulls off his sweater at a rehearsal and becomes Stanley, Helene is awe-struck and smitten. Demme films this pivotal moment using the zoom-in, track-out shot he previously borrowed from Hitchcock ("Vertigo") for his "The Last Embrace" (Spielberg also used it in "Jaws"). It makes people look as though they're being propelled forward from the background, and it's the perfect note on which to begin Helene's circuitous courtship of the mercurial and reclusive Harry.

"Once a play is over, whatever you thought Harry was just evaporates into thin air," says another member of the acting company, but Helene is made of stern stuff, and Sarandon is awfully good at conveying the powerhouse beneath the diminutive exterior, just as Walken enigmatically communicates Harry's daffily split personality. They're an endearing pair of fringe-dwellers, they are, and Demme has found the ideal bemused tone for their small-town romance.

Perhaps one is obliged to say the story is slight, but it's appealingly slight; it's slight in a major way. The music, by John Cale, is in the same sweetly winsome mood. About the only justifiable complaint is the use of a foolish "Sense of Humor" graphic at the outset, apparently to notify the ignorant viewership that what follows is lighthearted. Do public TV producers think the public is as dumb as commercial TV producers do? If so, help!