THEATER REVIEW

Audience plays the role of jury in  ''Witness for the Prosecution''. And the winner by the wide margin, for the most comfortable seat theatergoing in the city can be found in London County Hall.

Veteran playgoers who wind up in that stately building's Council Chamber, where a site -specific production of  Agatha-Christie's ''Witness for the Prosecution'' has become a novelty hit, may experience the giddy sensation of being upgraded from coach to business class on a trans-Atlantic flight.

Of course, there's much to be said for the venerable theater of the West End, where's it possible to envision the ghosts of centuries-old dames and dandies treading the aisles.

But such atmosphere comes with a price of sitting with your knees entirely too close to your chest and an uneasily enforced intimacy with people who, no matter what their actual dimensions, always seem overbearingly large.

The County Hall, on the other side of the Thames, was originally designed as as a center for civic government.

{It opened as such in 1922, but had shed its official functions by 1990.} Its central,  marbled-walled octagonal chamber features luxuriously padded  ''benches,''  divided into self-contained spaces with their own little desks-

Where the members of the Greater London Council might have nodded off during the debates among bloviators.

What better place in which to cozy up to the  Queen of the Comfort Mystery, Agatha Christie? The eternally best selling author, whose by-the-number crime fiction seems as likely to go out of print-

As the work of  Shakespeare, has long been an author that the anxious and the addled reach for in times of high stress.