After several chapters of blandly engaging buildup, aging flirt Sisu Potter--who's playing a small role in the amateur Carmel Playhouse production of A Classic Case of Murder--is found drowned in the Carmel River, following the cast's opening-night party. The obvious suspects? Well, Pea Brown--wealthy, promiscuous matron-about-town--hated Sisu for upstaging her in the show. Two male cast members--straying priest Ted Reid and proper lawyer Hilly Lawton--are among the dead woman's ex-lovers. And talented teen-ager Edwinta Everett (who belongs to one of Carmel's great, scandal-ridden clans) was obnoxiously snapped at by catty Sisu. Even by light-mystery standards, then, this is awfully thin stuff. So first-novelist Littlepage tries, not very successfully, to fill things out--with the Everett family's secrets, with the budding romance between leading-lady Vivienne Montrose (a former Hollywood star) and Carmel's widowered police chief. And there's mostly just talk--gossipy, repetitious--until the showdown (in a backstage vault, mid-performance) between quasi-heroine Vivienne and the ruthless, weakly motivated killer. Without the humor or pizazz that other writers have brought to murder in a theatrical setting: a harmless, only faintly diverting debut.