WHEN THE AUDIENCE files into the Old Library, the set for The Cradle Will Rock consists of only one object--an awkward cardboard-like streetlamp. The lights dim to blackness, a jarring piano theme begins, and quite suddenly the streetlamp switches on, illuminating a prostitute leaning against it. It is the first of many delicate and imaginative effects in a double-bill evening that provides more food for thought than most troupes could gracefully handle.

Throughout this long evening (each one-acter lasts more than an hour, and an extended break pushes the final curtain past 11:30) the twin productions maintain a sharp direction and pace that keeps them from flagging. Time speeds up and slows down often in the space between 8 p.m. and midnight; cynicism becomes hope and then a starry-eyed idealism inviting scorn, reality advances and recedes through a spyglass of jingoist jargon and lovers' quarrels. On the surface, the two shows--a self-styled "political allegory with music" and an original drama about a suicidal writer--could hardly have less in common. But they share a propensity for mind games, whether political or emotional the audience is teased into involvement, then thrust back out by a line or a cadence or the twist of the plot.

In The Cradle Will Rock, the first offering, the duplicity lies in the strange shadows the subject matter casts. Written by Marc Blitzstein in the depths of the depression. Cradle paints the struggles of infant unionism through a severe but jazzy stylization. The single piano hammers in the background as the residents of "Steeltown, U.S.A." battle the manipulations of the inexorable, cigar-chomping "Mister Mister" (David Reiffel). Mister Mister owns the factories and the town newspaper and heads the union-busting Liberty Committee, his wife bribes the preacher to fan war hysteria so steel prices will stay high, while his henchman track down dirt on the heroic labor-organizers who are trying to bring about a social revolution. Their speeches are not just background color thrown it to give characters an excuse for passionate devotion or to propel a romantic plot Rather they are the lyrical heart of the script, bringing intensity and believability to characters who otherwise conform with relish to the movement's righteous stereotypes

Which is where the mind games come in because, of course, it is impossible to watch such intensely political statements and scenarios unfold without drawing rather forlorn comparison between their unquestionable moral stance and the tangled feelings established unionism evokes today. Program notes from the director insist, a little stridently, that "Cradle is not a period piece." and make a valiant attempt to focus on the play's anti-jingoism as an issue still desperately topical in the 1980s. But it doesn't really work, because the political issue gives the play its power; no spectator could respond emotionally to Larry Foreman (Michael S. Miller) putting his life on the line for the revolution, or a bedraggled two-days-a-week girl turned hooker (Thania Papas) listening to him raptly as he raves in night court, without thrilling to the specific outrages of 1930s labor abuse.

The audience does thrill to it. Rhetoric though they be, Miller's fiery prophecies of revolution, Reiffel's well-padded complacency and the workers' vignettes of woe create a momentum that overcomes the pragmatic 1980s assumption that "nothing is ever that simple." Director Josh Milton's fine sense of timing and placement melds the difficult mancuvers of lockstep group motion and robot-like dance rhythms to reinforce a visceral feeling of brewing social pressure, of the inevitable coming explosion.

All in all, revolutionary fervor mounts high enough to lend credence to another program note on the play's history, which has become a theatrical legend: Originally written as a Federal Theater Project, Cradle was censored by its federal "employers," who refused to allow the actors to set foot on stage, so the pianist played while actors spoke the lines from their seats in the house. There are rough edges in this production, notably the awkward casting of Lars Gunnar-Wigemark as two very different characters in back-to-back scenes, and roughest among them remains the attempt to square things with the present. But the illusion holds, strongly enough so that it would hardly be a surprise if an audience member or two stood up to do his own yelling.

PERSONAL AND SEXUAL politics, rather than corporate ones, tease and distort the audience's expectations in the evening's second play, Bonnie Salomon's Who's the Fool Now? which takes its inspiration from the true story of a writer who commits suicide. Anyone expecting a letdown from Cradle's shuttering close should be not only pleasantly surprised but downright rolled up by a play that is satisfyingly complex and refreshingly free of the problems to which first plays by undergraduates are liable to fall prey.

The lights rise on a fairly elaborate living room, cluttered and homey No streetlamps here: the idiom is absolutely current, the conversation of two writers in their early 20s grounded in references so familiar and accessible that they occasionally give one pause. The writer, John Monroe (Kevin Porter), is an Amherst dropout writing a novel. A former girlfriend wanted him to go back to business school, he tells Natalie, the lover who narrates his story, but the relationship went nowhere Natalie. (Pamela Thomas) has graduated and now works as a secretary in the publishing house where John hopes to submit his book. Nothing is exotic or stylized about either the situation or the characters, who dress in the same kinds of clothes you just saw at intermission, and speak like your roommates. Salomon has relied on no easy gimmicks; everything must stand or fall by the writing, the construction, the plot.

And stand it does--for Salomon, it soon becomes evident, can really, truly write dialogue. The continuing repartee between Natalie and John, besides revealing humor and a fine ear, clearly delineates a couple of unusual characters. Porter, the stronger actor, fashions a strikingly individual Johnny out of scenes and speeches that often border on the lyrical. There are jokes with a personal stamp, characteristic expressions, a characteristic cadence. After he has left the outraged Natalie for two days, he returns to her with a speech about a pool game that evokes both a concrete scene and a mental state with startling insight. Sure, there are some problems. A few scenes too many end with a shouted "Oh, go away!" or a slamming door. The character of Natalie seems nowhere near as complex as Johnny's so that the story she narrates occasionally seems to escape her comprehension. The flashback monologues that tic the play together are its weakest stretches, whether because of the lines--which tend to spell out undertones that should be shown--or Thomas' occasionally uninspired reading of them. But such flaws are unimportant alongside a plot which, astonishingly, justifies not only the play's occasional lapses but its considerable length.

One senses these days that the Leverett House Arts Society knows its business. The hallmarks of a growing professionalism are evident from the start: Ushers are casual, handling the doorway crush smoothly, and the programs with their predictably flippant bios and nostalgic senior testimonials evince a comfortable style. The solid walls of the Old Library--one of the few legitimate full-size theaters available to House dramatists--reflect the assurance needed to knit two such diverse shows together and make them work. Small wonder, perhaps, that what happens on stage this weekend echoes such firmness.