THE BAT-At the Olney through September 16.

The lights flicker and fail, leaving the stage mottled with eerie shadows. Thunder cracks onimously. Veteran members of the Olney's audience are seized with fright -- not the fear that a grisly dramatized crime is about to take place, but the fear that one is not, at least until PEPCO restores the power.

What is brought to mind by the simulated weather conditions that herald murder in the current production, "The Bat," is how firmly set in conventions the theatrical murder mystery is. There is very little real difference between "The Bat," the 1920 hit by Mary Roberts Rinehart and Avery Hopwood, and this year's new hit, "Deathtrap," by Ira Levin.

Storms rage noisily, putting out the lights of wood-paneled rooms at crucial moments. Relatives and friends cannot be trusted. Jokes skillfully set off the horror. Metaphorical cracks about killing people are constantly made by the apparently innocent. Foreigners are automatically considered hilarious. Dead people can never be counted upon to stay dead.

Perhaps it's an ideal form, and needs no further development, although some details will eventually have to be modified. All that paneling, for instance, becomes increasingly harder to explain in picture-window America. Levin's hero is a Connecticut writer, so he has a fashionable excuse for living in an old cottage. As the foreign-born are increasingly apt to take offense at being ridiculed, it is safer to use northern Europeans for the purpose than Third Wonders. "Deathtrap" has the taste to use a Dane for comedy, where "The Bat" tastelessly has what is referred to as a "Jap."

The general sameness does mean, though, that the backlog of such plays can be recycled with as much success as new examples. Olney's production, directed by Leo Brady, approaches "The Bat," as if it were current, not going in for a campy atmosphere, but not being particularly creative about it, either. The chief slip-up is that the assortment of close associates who always need to be together in the library when the lights go out have, in this show, an international variety of accents.

Pauline Flanagan, as the brains of the place, and Jean Schertler, as her nervous maid, do their set characters very well. Pat Karpen, as the rattled ingenue, is so entirely rattled that one wishes for out-of- period tranquilizers. The others simply play their not-brilliant types as if they had been doing them, in one such play or another, for years, and knew better than to try developing anything beyond a certain programmed humor. That the dumb detective and the snobby college man are still so familiar as to be only mildly funny, but the Japanese butler joke has been shelved long enough to seem zany, cannot really be held to the blame or credit of the actors.