SIX IDIOSYNCRATIC guests snowbound in a manor house outside of London with their nervous host and hostess, an unsolved murder mystery, the haunting refrain of a childish nursery rhyme--these are the ingredients Agatha Christie uses to bait The Mousetrap. A lot of people have been snatching up the bait; the show has broken box office records, running 23 years on the London stage.

For Agatha Christie, murder reduces to an elegant picture puzzle, which audience and characters simultaneously try to piece together. When the overintense detective in The Mousetrap insists that "murder isn't fun and games," he's clearly wrong, and Evangeline Morphos, who directed the Leverett House production, fortunately knows it.

Morphos directs The Mousetrap so as to wring the maximum possible number of laughs from a fairly silly script. In the process, she necessarily subordinates the unwinding of the plot to the peculiarities of the characters who inhabit Agatha Christie's strangely isolated world.

There are definite problems with this approach. For one thing, Morphos must sacrifice some suspense on the altar of comic effect, and as a result, the final revelations lose part of their force. In addition, her emphasis curtails the range of emotions implicit in the script, since characters as different as the cynical Mr. Paravicini and the pathetic would-be architect Christopher Wren emerge in this production as similarly successful comic types. Sometimes laughter intrudes where it shouldn't; for example, Wren's paranoid outburst in the second act ("You're all against me, everyone's always been against me"), is in context far more amusing than pitiable.

Nevertheless, a less light-hearted production of The Mousetrap might well have lapsed into maudlin sentimentality or self-importance. Agatha Christie just can't be taken too seriously, especially since in this specific play, she is poking fun at the conventions of the murder mystery genre, including her own work. At one point, for instance, she has Detective Sergeant Trotter--himself an insane parody of crime-fighting zeal--ask the other characters to reconstruct the crime. "Oh, that old chestnut," Mr. Paravicini sneers.

At the very least, then, this Mousetrap entertains, in large part due to the exertions of a talented cast. The first act rushes breathlessly by, with each character in succession making his appearance and promptly revealing his own brand of madness. While the quickness of the first act sometimes seems forced, the natural momentum of events whirls the second act on to a satisfactory--if not stunning--conclusion.

THE STANDOUTS in a generally fine cast are Davis Goodman as the effeminate Wren and Sam Bloomfield as Mr. Paravicini. Goodman minces marvelously through his role as the child-like homosexual, and Bloomfield, clad in an elegant dinner jacket and bedizened with rings, gives a superbly controlled performance as the uninvited guest. Also good is Mark Howard, appropriately manic as Detective Sergeant Trotter. Nancy Abrams makes a stony-faced Mrs. Boyle, although her carefully accented syllables sound too much like metered poetry.

The Mousetrap is staged in the Leverett Old Library Theater, a sort of "theater in the square," where seats flank the stage on four sides. Careful blocking, however, keeps the audience from spending too much time contemplating actors' backs.

The world of The Mousetrap is peopled with immature, often exaggerated characters, who rationalize their idiosyncracies by suggesting that "perhaps it's fun to be crazy." The mark of this production's success is that it half-persuades us to accept this premise, if only for the night.