DISCRETION is the better part of valor, but it is usually considered the lesser part of theater. What's drama for, if not a flailing of arms and a gnashing of teeth? To withhold the full range of emotions is, presumably, to stint on the merchandise, isn't it? Since it all began with the Greeks, the theater, to borrow a term from the M60s, has been in the business of letting it hang out.

That's why A.R. Gurney Jr., the author of the recent Eisenhower tenant "The Dining Room," and now a new drama, "How I Spent Last Summer," may be one of the more interesting playwrights working in the American theater today. Gurney doesn't ask his characters to fling open their souls and bear their innermost secrets. In one respect, he is merely being truthful to his constant subject, which is the well-manicured WASP world of northeastern America. He knows that its inhabitants, basically reticent by nature, cannot be forced into high-blown drama. "Don't make a scene" is the cardinal rule of WASP society, and a playwright would have to be mighty obstinate indeed to change a couple hundred years of ingrained behavior.

But being something of a button-down WASP himself, Gurney also has an estimable respect for privacy. In an age of snoops and gossips, he refuses to pry or meddle. He's perfectly content to observe the patterns of his characters' lives and the persistence of their social rituals, until a rift opens up and he catches them in a momentary flurry of indecision or self-doubt. As a result Gurney's plays are not "big" affairs. They approach drama tangentially, from the sidelines, instead of meeting it head-on. But they are written with warmth, wry humor and unfailing accuracy of detail.

The heartening success of "The Dining Room" -- off-Broadway, here at the Kennedy Center and, according to all indications, in San Francisco, where the Kennedy Center production has set up temporary shop -- suggests that Gurney may be providing a welcome antidote to the rampant confessionalism that poisons much of our reading matter and television fare these days.

He has not changed his stripes with "How I Spent Last Summer," which has been undergoing a circuitous pre-Broadway tryout this month that has taken it from Cape Cod to Colorado. Once again, he is chronicling the growing pains of a particular WASP family -- probably his own -- vacationing on Lake Erie. The father is away on a destroyer in the South Pacific, fighting World War II, and the mother (Barbara Feldon) is holding on by her fingernails as she attempts to keep up a semblance of normal family life. Her two children are fighting battles of their own -- those of adolescence -- and the play concentrates on 14-year-old Charlie's bid for independence.

It starts out simply enough when he takes a job, 25 cents an hour, doing odd chores for Anna Trumbull (Eileen Heckart), an eccentric neighbor whose patent disregard for convention has earned her the nickname of the Pig Woman. To the general horror of the local uppercrust, Anna doesn't wear a top when she gardens or a bathing suit when she swims, boasts openly of her Indian blood, and will tell anyone who asks, and some who don't, that lawns are an integral part of the class system. ("Wherever there's grass, there's class.")

There's a bit of Miss Jean Brodie in Anna as well, and she prides herself on being able to tap the secret essence of any youngster who falls under her spell. Charlie does. "All great teachers are dangerous," she tells him blithely, "such as Socrates, Christ or me." What she really teaches Charlie, as she's setting him up with a basin of modeling clay or lecturing him on the wonder and tradition of "the noble tomato," is to question some of the staid and strangulated assumptions of WASPdom. Charlie may not be the free spirit she takes him for -- he is, like most adolescents his age, a handful of insecurities -- but he will probably become, in years to come, a far freer adult because of her.

"How I Spent Last Summer" moseys through the events of the summer, both ordinary and extraordinary. Each character gets the chance to talk directly to the audience and explain why he or she thinks the play is really "about me" -- except for Charlie's teen-age sister, who, feeling typically under-appreciated, is pretty certain the play is not about her. Charlie's mother tries to reclaim her son from Anna's clutches, and drifts in her frustration into a brief off-stage fling with a man down the beach. Charlies's friends razz and jazz him. A late-night trip to the amusement park with the "third-largest roller caster north of the Susquehanna" brings the summer to a precipitous close.

This slice of life has been neatly staged by Melvin Bernhardt, who takes events as they come and wisely refuses to make a big fuss over them. Fuss enough comes from Charlie, who seizes upon his new insights somewhat the way Joan of Arc seized her battle standard. (Mark Arnott's performance is pubescent-perfect and should be part of any future productions.) If the play invites one criticism, it is the charge normally laid at Gurney's feet: namely, that his writing is essentially anecdotal in nature. It is true that "How I Spent Last Summer" doesn't really build to an earth-shaking climax. I'm not sure it matters. What unfolds, instead, is a tapestry -- honest, decent and often funny -- of people coping.

No writer worth his salt believes that his work is modest. If he did, he wouldn't bother. But it's the apparent modesty of Gurney's plays that makes them, in a theater of flash and din, such a refreshment. We live in a time when closet doors have been seemingly yanked off their hinges by people eager to get out and blab their raucous tales. We tend to forget that the true confession is, more often than not, mere exhibitionism. What Gurney tells us about his characters, he tells us indirectly, by suggestion and implication.

To Broadway eyes, this may make the play trivial. Broadway eyes, after all, are trained to look for the exploitable commodity. However, I saw the play one recent star-lit night on Cape Cod. The sun had blazed all day, but by nightfall a breeze was coming in off the ocean, crickets were chirping in the pines and soft light was spilling from the windows of the tidy saltbox houses.

In that peaceable context, "How I Spent Last Summer" looked a lot like real life to me.