FOR A few days last weekend, Louisville, Ky., became the theater capital of the Western world.

Producers, directors, agents, playwrights, scouts and critics by the score -- from Washington, New York City, Minneapolis, Nigeria, Poland, Australia and such formidable feifdoms as the William Morris Agency, Universal Pictures, the Kennedy Center, Arena Stage, 20th Century Fox and the New York Shakespeare Festival -- had packed their toilet kits and flown in for a weekend orgy called the fourth annual Actors' Theatre Festival of New American Plays.

The idea was to sit through nine productions between midday Friday and midnight Sunday -- but anyone whose heavy schedule barred a three-day getaway could cram the whole package into Saturday and Sunday alone.

Coffee was served -- often -- and "I Survived the Actors' Theatre Festival" T-shirts were bestowed on all who could wear them honestly.

Founded in 1964, now housed in two recycled commercial buildings a block from the Ohio River, the Actors' Theatre has become the overnight sensation of the resident theater world since producing-director Jon Jory took the helm in 1969. Although not quite ready to supplant the Kentucky Derby as a tourist attraction, the theater places high on every list of Louisville landmarks.

The festival began with two plays in 1977. One was D. L. Coburn's "The Gin Game," which captured the Pulitzer Prize when it advanced to Broadway a year later. The 1978 festival grew to six plays, and the standout his was Marsha Norman's "Getting Out," which ran for six months off-Broadway, won awards of its own and is likely to be produced here by Arena Stage next year. In 1979 there were six full-scale productions and three workshops, and James McLure's "Lone Star" went on to be produced in New York, Baltimore and Princeton.

The Actors' Theatre has not built its national reputation at the expense of its local following. The theater has subscribers enough to fill 96 percent of the seats in its main auditorium, and box-office income enough to defray 70 percent of its expenses -- both unusually high figures for a resident theater. (Arena Stage's comparable statistics are 50 percent and 69 percent; the Folger Theatre's are 65 percent and 55 percent.)

With every year's festival, the influence and number of out-of-towners have swelled, and the auction-block side of the enterprise has become more open. Not only plays but also authors, actors and directors are up for grabs. Humans and scripts alike are talked about by some guests, in terms usually reserved, around Louisville, for horses.

At the festival, some of the talking took place at a vast Saturday night party hosted by Barry Bingham Jr., a theater board member and editor and publisher of the Louisville Courier-Journal. And some took place at Sunday night's going-away party, held in the slightly more modest confines of the theater's basement bar/cabaret, The Starving Artist.

"I wouldn't get involved in any actual bidding right here -- the prices are too inflated," said a yound Broadway producer, trying to carve out a little talking room behind a pillar. But he went on to intone, with effusive self-confidence, that among the eight new plays he had just seen, only one was destined for glory: "Agnes of God," about a nun accused of murdering her own newborn child. A few weeks hence, the producer hinted, when the festival frenzy had died down, he would be talking to the playwright and his agent about doing "Agnes of God" on Broadway.

Another visitor, meanwhile, was declaring with equal certainty that the most promising play of the festival was "They're Coming to Make It Brighter," a comedy about three elevator operators and a shoeshine man in a big New York office building. He was a top executive for a major movie company that has bankrolled several Broadway shows, so his opinion counted too.

That, of course, is what makes a horse race.

Just which of the 1980 festival's plays are most likely to turn up prominently a season or so hence may be debatable. But if recent history is a guide, one or more surely will. And beyond the plays themselves, the festival is an event that reveals patterns of form and theme that may come to be characteristic of the American theater generally.

At birth, an aberration and a trend can be as tough to distinguish as a frolicsome pair of twins. But just the same -- and without any speculation as to their life-expectancies -- here are a few of the developments observed flexing their infant muscles in Louisville last weekend:

Plays written by actors: Three of the festival productions -- Kent Broadhurst's "They're Coming to Make It Brighter," John Pielmeier's "Agnes of God" and Ray Aranha's "Remington" -- are by actors who have appeared in Actors' Theatre productions. The actor/playwright tends to make fewer mundane mistakes, according to Jon Jory. "You don't have to go to them and say 'I'm terribly sorry, but they can't possibly make the costume change you have written.'" More importantly, an actor doesn't overwrite, Jory says. "The great problem with young writers is that they write too much."

An extra dividend for the actor/writer is the chance to give himself a memorable role. Broadhurst has an extremely funny cameo in "Brighter" as a prissy young man who has just had his coat destroyed in a run-in with a truckdriver. Holding the riped remains in his hand, he sighs and notes philosophically: "It's just matter . . . It was just passing through my life."

Plays about everyday people: In the 1960s and '70s, American playwrights tended to create characters who floated free of any particular time and place.Jory blames the influence of European models -- Samuel Beckett, Harold Printer and Eugene Ionesco, for example. "The rhythms of American plays were so ponderous," says Jory. "A leaden seriousness decended on the American playwright. Now it's possible again to detect in serious plays the American sense of humor."

At the festival, this resurgence was most apparent in "They're Coming to Make it Brighter" and David Blomquist's "Weekends Like Other People," both a lot snappier than their longwinded titles, both about working folks at the ignorant foot of the corporate ladder. "Brighter" takes place in the lobby of a grand old New York City office building, presided over by three elevator operators and a shoeshine man -- all black and all outwardly subservient toward the building's important tenants. The play, which is not long on plot but offers a large dose of humor as a substitute, might be subtitled "Uncle Tom's Skyscraper."

"I think the monotony of the work is a pain in the ass," says one of the elevator men, "but I rise above it. I rise above it. Get it?"

(Broadhurst, the actor/author of "Brighter," is white. But Aranha, the author of "Remington," a one-man show about the 19th-century Western artist -- who was white -- is himself black. And "Today a Little Extra," a play about two elderly Orthodox Jews, has a black director, Israel Hicks.So at least on the evidence of its new-play festival, the Actors' Theatre has achieved a remarkable degree of indifference to race, which may turn out to be another exportable trend.)

Whatever its theatrical future, "Brighter" seems virtually guaranteed to inspire a half-hour TV sitcom. "Weekends," with Pat McNamara and Carol Teitel as an average American couple who aren't altogether happy with their averageness, starts out like a sitcom but gradually turns more complex.When Teitel starts worrying about the cancer hazard of hair dyes, something she has heard discussed on TV, McNamara tells her, "Don't believe that. That's just advertising."

A more low-keyed play, Shirley Lauro's one-act "Coal Diamond," is about a group of women playing bridge on their lunch break. And it is about the evils of telling stories behind someone's back. A humdrum group of characters and a humdrum subject, but sharply explored.

Plays about materialism and progress-gone-amok: The plot of "Brighter" -- what there is of it -- involves the replacement of three spectacular and perfectly functional Art Deco light fixtures with three hideously modern white globes -- all arranged by absentee conglomerate landlords.

Adele Edling Shank's heavy-handed but interesting "Sunset/Sunrise" paints a frightening Sam Shepardesque portrait of upper-middle-class, southern California decadence. The ambitious set, depicting the exterior and some of the interior of a gadget-ridden house, includes a vintage 1951 Pontiac and a genuine working hot tub. One of the characters is a woman so allergic to so many different things that she lives in a sterile room and communicates with the outside world -- and the audience -- only over closed-circuit TV.

Michael Kassin's "Today a Little Extra" -- a tediously cliche-ridden work, unfortunately -- is about a kosher Jewish butcher who agonizes over the fate of his shop after his retirement, fearing his successor will go non-kosher.

Plays about the cultural gap between coastal, sophisticated, Freudian-minded Americans and their heartland brethren: The second of Shirley Lauro's two one-act plays -- a crude effort compared to the other -- is a two-woman variation on "Psycho," with a New Yorker trapped in a smalltown motel with the manager, a Bible-and-rifle-wielding fundamentalist. The manager holds her guest accountable for the entire breakdown of American morality, and accuses her, what's more, of spreading lies about Richard Nixon, thus lowering him in the eyes of his daughters. "Them two pretty girls just about broke their hearts over their daddy," the motel manager exclaims.

"Agnes of God" has a psychiatrist interrogating, on the one hand, the young nun who claims not to remember having a child, or strangling it, and, on the other, a mother superior who believes the birth was divine and shouldn't be probed too deeply. The idea of science and the supernatural in a standoff is not the most original notion, and the play seems ill-thought-out rather than merely ambiguous. But its commercial allure is beyond question. Murder, scandal, mystery, an atmosphere of profundity, three characters, minimal set -- if there are any sure-fire formulas in the theater, "Agnes of God" seems to be onto one.