THIS is an unlikely theater town, full of unlikely theaters. The audience is incongruous: Down an otherwise dark alley in a seedy part of Hollywood, behind a strip joint, a little crowd of the Mercedes-and-Volvo set - miles from home - mingles quietly over styrofoam cups, waiting for curtain time inside. There are similar clusters all over Los Angeles nearly every night.
The actors are unlikely: Many of them are familiar faces from the television screen, generally working for little or no pay in theaters the size of modest class-rooms.
The whole, teeming, theater culture here - growing up in the low-rent corners of this glamour-bound city - is incongruous.
After all, it's a long way from here to Broadway (fame), and just a short drive to the Polo Lounge (fortune).
In other words, New York is still America's theater town, where the stage is taken seriously and reputations are made. And Los Angeles is still Tinseltown, with an eager pool of some 40,000 actors, most of whom will drop a stage role the moment a movie or television camera pans their way.
But in the past 10 years, an ambitious theater scene has been built in Los Angeles that, by local accounts, produces more plays each year than Manhattan.
Take the enterprising Bob Bushnell, producing director of the Los Angeles Actors Theater. He rents offices for $1 a month over a bank in a run-down part of Hollywood. His two stages are down an alley across the street.
From here, he is raising the money - and is busy selling season subscriptions - for an august, $13.6 million, four-theater, two-restaurant complex with art gallery and bookstore in the heart of downtown.
Or look at Ron Sossi, artistic director of the Odyssey Theater Ensemble. His three theaters are housed in a former scuba-diving school on a busy corner in west Los Angeles.
From here, he is in the middle of assembling a board of leading citizens to assist him in building a ''world class'' experimental theater center with an 800 -seat house, a 500-seat house, a cabaret, and some living quarters for Thespians from abroad.
Mr. Sossi envisions ''sort of a giant think tank'' to stand as the centerpiece of the downtown Santa Monica redevelopment effort.
Bushnell and Sossi are two of the handful of impresarios here who have established a reputation for quality in small theaters and are restless for bigger things.
Los Angeles has a fair number of big theaters (houses with upwards of 1,000 seats) and a thriving mob of little theaters (less than 100 seats). But it has a dearth of the midsize houses that are big enough to be commercially successful but small enough to take artistic risks.
The midsize house, says Susan Dietz, who runs two of them successfully, is going to be ''the movement of the '80s in L.A.''
Ms. Dietz cites several projects rumored to be afoot for building new playhouses with seating capacity in the middle range. She is skeptical, though, as are others, that all this theatrical bullishness is justified. ''We may have outstripped our audiences,'' she says.
What apparently hampers play attendance here most is that Los Angeles theaters don't get the tourist trade that keeps Broadway in business and cushions San Francisco receipts. Both tourists and theatrical hopefuls are slow to see Los Angeles as a theater town. ''There's still the New York myth,'' according to Sossi.
Joseph Stern, a theater producer here who acted in some 80 off-Broadway plays before moving West, sees a reluctance by the Eastern theater establishment to acknowledge that ''in this glossy, mogul-filled town there is something entirely different going on.''
Sossi reckons that, subtracting tourists, Los Angeles has as big a theater audience as New York's. Playgoers in New York are more sophisticated, he says. They've seen more plays. But they can also be more jaded and prone to fads.
''I like this audience,'' he says. ''I have a feeling it's kind of like Shakespeare's audience - a real mixed bag.
''We're more heart- and gut-oriented, more daring in a reckless sort of way. . . . It's not a knit-brow kind of thing. It's not 'hmmm,' '' he says, striking a thoughtful pose. ''It's more 'wa-a-ah!' '' he says, throwing his arms open.
There are about 150 theaters in Los Angeles with fewer than 100 seats each. They owe their existence to the ''equity waiver'' rule by which the union allows its members to work in these tiny houses for little or no pay.
They are spread out in typical Los Angeles fashion so that, as Bushnell jokes , it is sometimes easier to catch a plane to a play in San Francisco than to get to a theater across town in L.A.
Patrons, although growing in numbers, are fickle. Still apt to go to a movie instead, theater audiences drop by 30 percent when it rains, another producer laments.
Yet theaters in Greater Los Angeles staged 600 professional productions during 1983, by a rough count.
That's quantity. As a hazy measure of quality, Susan Dietz recalls that five years ago she had a terrible time getting literary agents to return her phone calls from New York. Even when she got through, she says, they wouldn't sell her the rights to promising plays.
All that has changed. Ms. Dietz is artistic director for Los Angeles Stage Company with midsize theaters in Hollywood and Beverly Hills. She has made a box-office success so far of bringing off-Broadway hits to Los Angeles. And much smaller L.A. theaters than hers, she notes, are getting rights to quality plays.
''Something heroic is going on here,'' attests Joseph Stern, producing artistic director of Actors for Themselves at the Matrix, an equity-waiver theater. Angelenos have built a serious, quality theater culture in the last 10 years, he says, in their spare time.
''They've had to hold other jobs,'' he says, because the little equity-waiver houses can seldom pay actors.
Mr. Stern himself augments his income as a TV producer on shows such as the miniseries ''Winds of War'' and ''Cagney and Lacey.'' His reputation, though, rests on the theater produced on the Matrix stage.
Ron Sossi has also tasted the other side of Hollywood. As an executive with ABC-TV working on series such as ''Bewitched'' and ''The Flying Nun,'' he grew so restless in the lucrative but limited world of mass entertainment that he started up the Odyssey ensemble in his spare time. Eventually he left ABC for his theater company, and it now has an established reputation as an avant-garde, experimental theater.
Theater in this city lives under the heavy shadow of the entertainment industry. ''It's a town that eats up talent. It eats up faces; it eats up writers. You can never get enough writers,'' says Stern. Would-be playwrights often wander away from theater at a trot when they can sell a TV script for $30, 000 or $40,000.
''It's a mixed blessing,'' Sossi says. ''I can get better actors than I could 10 years ago (when local theater was less respectable), but I could lose them overnight if a movie part comes along.''