Charm dances along a knife-edge. When it succeeds, nothing is more charming, but if it slips, the fall is acute.

That may seem overly dramatic for something as slight as "The Apple Tree," a 1966 trio of one-act musical snacks by Jerry Bock and Sheldon Harnick. But slightness makes charm all the more crucial.

Slightness can be charming in itself, I suppose, and a recent Theatre Factory matinee audience responded happily. But it takes skill to succeed on performance alone. What's odd is that the trio uses material by three famed humorists, Mark Twain, Frank Stockton and Jules Feiffer, which suggests more substance than appears.

I was thoroughly on board through the first and longest of the trio, "The Diary of Adam and Eve" (based on Twain), in which diarist Adam (Michael Misko) shares the bewilderment, frustration and eventual affection stirred by the arrival of the inexplicable (to him) Eve (Amanda Slaughter), exacerbated by the interference of a rather stolid snake (Michael Canali).

Adam's bewilderment is funny and Eve's bright enthusiasm is absolutely charming, as embodied by the deft, sweet Slaughter. But even here charm has its limits, as the story comes to an abrupt end, unwilling to deal with the dark implications of the expulsion from Eden.

Stockton's parable is "The Lady or the Tiger," in which a balladeer (Canali) tells of a distant princess (Slaughter) who has to decide whether to save her lover (Misko) from her tyrannical father (Jason Barnsley) by delivering him into the hands of the lovely Nadjira (Deborah Bender) rather than a hungry tiger. What should be charming or chilling or at least ironic falls pretty flat.

"Passionella" is one of cartoonist Feiffer's wry little parables, in which a chimney sweep's (Slaughter) fantasy of movie stardom leads to an ironically happy ending with a prince charming (Misko) who turns out to be ordinary, just like her.

"The Apple Tree" originally ran for a year on Broadway, buoyed by the charms of Alan Alda and Barbara Harris. But the score, although played energetically by music director Janelle Garoff, is pedestrian, and director Elizabeth Matthews isn't able to sustain the whimsy the show needs.

At Theatre Factory, Cavitt Avenue and Third Street, Trafford; tonight 8 p.m., Sun. 2 p.m.; $16-$18; 412-374-9200.