PEOPLE WHO started out thinking of Stephen Sondheim as just a clever lyricist have long ago given him his due as an artist--and a chameleon. As soon as he started writing the music to go behind his own words. Sondheim began varying the roles he could play--sliding a melody off key or twisiting a double-entendre into mid-line, he was the vitriolic social critic in Sweenes Todd, the light satirist in A Funny Thing Happened On the Way to the Forum, the gentle troubadour in A Little Night Music; he was blandly anthologized as cultural phenomenon in Side by Side by Sondheim. Sondheim's tools are unquestionably the right ones: a versatile versifier can be anyone.

The Sondheim-as-Jacques-Brel image to which Marry Mc A Little pays homage is among the more limited of these roles, perhaps because it is set so firmly in one particular milieu. The songs that make up this essentially plotless revue about a man and woman--each alone on a Saturday night in the same apartment building--could not possibly refer to any society but that of upper-middle-class singles in Manhattan. There are references to reading the Sunday Times, or Saturday night prowling various bars, putting up with parents and their querulous worries about their kids in the Big City. And there are the songs themselves, with their laments about loneliness, their fantasies ranging from the crude to the delicately romantic, and their general paean to a life of high-powered lack of direction. "Let me tell you one thin," one audience member was overheard muttering on the way out, "that's just not the way it is in the suburbs of Detroit.

Stringing 19 of Sondheim's upper-Manhattan-loneliness-love-songs together with only a sketchy, if provocative, plot does not seem to be a fundamentally good idea. Which is not to say that the songs aren't clever, funny, and tuneful, or that their presentation in the Leverett Old Library leaves anything to be desired in terms of theatrical technique. On the contrary, the difficulties in structure often have the effect of creating more levels of meaning, especially with two extremely skilled performers there to explore those levels.

THE PROGRAM NOTES state only that the two protagonists--a man (Ernie Kerns) and a woman (Jacqueline Wetss)--live in apartments 2E and 3E of the same apartment building, but have never met. This coincidence becomes not a plot device--as the audience keeps expecting for the first five songs or so--but rather as a springboard for various types of fantasy. Being in the same apartment line, they are of course identical, a detail which makes possible a fruitful theatrical device: the two characters play out their separate alonenesses simultaneously on the same set. Most of the dramatic tension, then, resides in the blocking, since both actors must studiously maintain the illusion of solitude while barely avoiding coliisions. The setup also gives the directors a concrete metaphor for the songs' philosophizing: when the characters notice each other at all, when they touch, the show crosses from reality into fantasy.

The songs themselves, though they never coalesce into any particular logical sequence, offer a good deal of emotional food for thought. Sondheim almost never gives into the simplistic. All 19 songs fall into the relationships genre which is his specialty, treating first thrill breakup and all the shades of happiness and disillusion in between. A few numbers like "Bang!", are explicitly sexual; others fit into a dreamier romantic mode, crooning, "Who could be blue when some-where, there is you?" Others are about frustration, repressed and not so repressed: In "Saturday Night," both singers declare together, "Alive and alone on a Saturday night is dead!"

THOUGH THE THEMATIC scheme doesn't seem to have been fully worked out (One song on golf seems particularly ill placed), the production does sketch a progression of loneliness, to fantasy, almost to fulfillment--and back again. More important, though, it affords a showcase for Kerns's and Weiss's extraordinary range. Dashing around the cramped apartment, the actors create a pair of convincingly frustrated personalities entirely through song, since no dialogue connects one number to the next. Weiss in particular has a voice that stays equally strong and flexible over several octaves, and her command of Sondheim's sliding harmonies is impressive. When she and Kerns sing love songs, the intensity level goes so high that the inevitable "someone" reference flips us back with a shock: what we're watching is not real, it's just another fantasy. Both singers reach their peak in the climactic title number, "Marry Me a Little," which explores fear of commitment and emands from outright belting to the subtlest possible tone modulations--tenderness, anger, and fear.

The pace never lags, supported by Richard A. Shore's brisk musical direction and David Reiffel's sophisticated, rapid-fire blocking. After 19 successive dousings in intense musical emotion, the audience probably couldn't manage more than the hour the show occupies. Marry Mc A Little isn't cathartic, and it doesn't leave the satisfying sense that Sondheim has worked out the problems of love; then again, no one would really expect him to. Instead, his insights are left vacue enough and universal enough to roll anybody up, and that makes the show as good a way as any to spend Saturday night--even alone.