ABOUT FOUR YEARS ago my friend Jonathan, who's an actor in New York, was out of work and a little depressed. One afternoon he went to visit another friend of ours, who invited him to stay for dinner and the reading of a new play by a young writer named Albert Innaurato. The play was called "Gemini," and Jonathan, who is quite slim, read the part of a fat teen-ager who was then named Marshall.

Now "Gemini" is playing on Broadway in a theater near Sardi's, and tickets are selling for months ahead. Jonathan (whose last name is Hadary) recently left the production after playing the part of the fat kid (whose name became Herschel) 750 times-650 of them on Broadway.

In between the reading and Broadway there were three other productions as "Gemini"-an ethnically spiced comedy about an Ivy-Leaguer's sexual conflicts-tap-danced it way from off-off Broadway to off-Broadway to the Great White Way.

The first rehearsals were held in an old office near Times Square, where there was no heat and the windows faced the news ticking in lights across the top of the Allied Chemical building. The actors were paid a total of $65 for several weeks of rehearsals and 12 performances. which were given at Playwright's Horizons, an off-off-Broadway theater respected for its productions of new plays.

The New York Times reviewed it, and several producers came to see it. "This guy may pick it up and move it to someplace in Long Island," Jonathan reported toward the end of the run-a conversation that was to be repeated in differing versions in the months ahead. Indeed, producer Jay Broad took it to the Performing Arts Foundation (P.A.F.) in Huntington, Long Island, where the actors were paid $200 a week and a round-trip commuter train ticket.

Then Marshall Mason of Circle Repertory Theater, another highly respected off-Broadway producer, opened the play at his theater, after redirecting it somewhat. It was popular, the run was extended, and then Mason, Broad and P.A.F. decided to "take it into the big ring."

At Broadway's Little Theater, Jonathan was making $355 a week and the food he had to consume onstage-jelly doughnuts and spaghetti, improved dramatically. On Broadway the spaghetti came from Mama Leone's, the play ran 15 minutes shorter, and "you had to be louder, face front more, and wear more make-up," Jonathan said.

"Gemini" is one of the Cinderellas of off-off Broadway. While some might argue that Cinderella looked better in rags than a fancy ball gown, "Gemini" serves to focus attention not on where it went, but from whence it came-the off-off Broadway theater, which may be the most energetic, productive and vital theater arena in the world.

The 200 or so theaters that can be called off-off Broadway range from atrocious to wonderful, and manage to hit all points in between. But the fact is that of late the noncommercial theater has been nourishing Broadway with successes cultivated in the lower-cost, lower-risk laboratory of off-off Broadway. Recently for example, over a third of the nearly 30 shows on Braodway originated off-off-and at other times the proportion has been even higher. There is even a 60,000 circulation newspaper devoted solely to activities of off-off Broadway called "Other Stages."

"There's more theater in New York than anywhere in the world," said producer Joseph Papp, "and its mostly off-off Broadway. There's a lot of junk. But at its best it is the kind of ferment that keeps the theater alive."

Not that Broadway is necessarily the goal of off-off Broadway, where producers and directors sometimes pride themsevles on being as noncommercial as possible. Indeed, some of the more experimental theaters, many of them well established over the years, cringe at the very though of Broadway's commercial slickness.

That shows as disparate as "Gemini" and "A Chorus Line" have been successfully moved to Broadway is merely an affirmation of the vitality and range of the off-off Broadway movement-and in a felicitous turnabout, the royalities that can be earned from a hit help finance the continuing work of the originating theater.

Papp said, for example, that $4 million of his Shakespeare Festival came this year from the profits of "A Chorus Line," which began as a workshop on one of his Public Theater stages.

Off-off Broadway shows are usually called "showcases" by actors, partly because they don't get paid much, if anything, and partly because they are seen as vehicles to "be seen." Once cast, an actor sends out flyers to all the agents and producers he can think of in the hopes of getting one of them to come and see him in something.

"I don't think about Broadway a lot," said Bob Moss, artistic director of Playwright's Horizons. "I don't go a lot because I can't afford it."

"As the production dates near you call up agents and ask if they got your flyer, and they say they don't know if they'll be able to come," said Mercedes Ruehl, and actress who recently appeared at one of the better known off-off Broadway theaters, Cubiculo, in a play called "Flight 981."

("Flight 981" was about a Turkish airlines jet that crashed, killing 364 people. She played two parts, one a designer who begins a romance with a race car driver on the plane before it crashes, and the other a woman waiting at the airport for one of the victims.)

"The next day you call back and ask if you can make reservations for them for a particular day," Ruehl said. "It's horrible, it's a very difficult thing to do. One of my friends copes with it by making all her calls lying down.

"Actors will do almost any showcase," she said, "you look at the script and you say, 'is there anything in this that could make me look good.' The producers and directors can afford to think about the philosophy of the theater; the actor has a very specific kind of hunger."

The term "showcase" also refers to the contract allowed by the actors' union, Actors Equity. Originally designed to protect actors from being exploited completely, the contract allows their members to work in non-paying jobs if the performances are limited to 12, the admission charged is no more than $3, and if the theater seats no more than 100 people. The idea is that the producer shouldn't be making money if the actors aren't. Most of the reputable, continuing theaters use Equity members, and the membership has been fighting with Equity management to relax the restrictions of the showcase code for about four years.

"What happened is that a few years after the showcase code was written, the whole not-for-profit scene started to develop; and instead of disappearing after one show, some of these companies started to survive," said Ellen Rudolph, executive director of the Off-Off Broadway Alliance (OOBA), a group of 90 theaters that joined together partly to spearhead the negotiations with Equity.

"As the theaters continued to survive, the union did not come up with any new document to allow for growth. It's changed from being primarily a theater created by actors to allow them to be seen or to try a particular part," see continued, "and become a real theater institution. Twelve performances are not enough."

Negotiations are again underway, and an agreement that, among other things, will allow theaters to advertise and permit up to 24 performances will probably be reached.

"Showcase is a deplorable term," said Papp.

"Showcase is an onerous term and implies a group of actors who get together to show themselves to producers and agents," said Bob Moss.

"You do a showcase because it's a good chance," said actress Carol Lynn Maillard, formerly of the D.C. Black Repertory Co., who is currently in a showcase by Micki Grant, "Isn't It Nice to Be Civilized," at the AMAS Repertory Co. on E. 104th Street. "If you don't have a particular face that can be packaged as a disco queen or whatever, that's all hear from producers or agents: "'Let me know when you're doing something.'"

"Off-off Broadway is committed to developmental theater," Rudolph said, "the plays are not chosen because they might be a hit. The theaters are not defined by one play, but by a season and a philosophy . . . There are a lot of questions I can't answere-why is the commercial theater looking to us? What has fostered the climate that has produced all this activity, and is it fostering better work or just a lot of work?"

That there is a lot of it, and a mind-boggling variety of it, is clear just from looking at the listings in the Village Voice (which gives the most complete guide for out-of-towners) or Other Stages (for the New York aficionado).

In one recent week, for example, there were 183 productions listed in Other Stages and 114 in The Voice. There were such enticements as the No Smoking Playhouse ("we call it that because we do mostly comedy and the name makes people laugh," said founder Norman Thomas Marshall.) They survive partly through the income from a thrift shop ajoining the theater.

"We survive by sheer cleverness," Marshall said, "And occasionally we rent the theater to someone else at a usurious rate."

Among the Alliance's members is a group called "Medusa's Revenge," about which information is not easily obtained. A woman who answered their telephone said in a Spanish accent she was just cleaning up the theater and didn't know anything about it.

Then there was a cabaret, "An Evening of Disgusting Songs and Pukey Images," at the Theater for the New City, and something called "Colorless Green Ideas Sleep Furiously" at the NYU Repertory Co. "The Fruit of Zaloom," might appeal to some or "The Dead Class," by the Theater of Poland at La Mama, the oldest of the off-off Broadway play-houses. A playgoer who saw "The Dead Class" said all the characters in it were dead, and not only that, they spoke Polish. "It was quite unusual," he reported.

The Interart theater, Rudolph said, "is not a feminist theater but they do only scripts by women and have only women directors and administrators." The Lion theater was started by actors who wanted to direct, and their gimmick was to be open in the summer when most other theaters were closed. They've been successful enough to operate all year.The play "Vanities," a tale of three cheer-leaders that has become a staple of theaters everywhere (including a Washington area high school, where parents in the audience were less than thrilled with the play) had its first production there.

One recent production at another off-off Broadway theater called "Andy Warhol's Last Love," featured two women sitting in a store-front window, "typing sulkily" and shivering obviously. One of the women wore her hair in a topknot with a phallic shape sticking out of it, giving a unicorn effect. Lights shone through the window, and passers-by would pause to stare in-until they realized that by so doing they became part of the show.

"There are some really weird productions," said Mercedes Ruehl, who once played the part of a crippled woman involved in "some bizarre quasi-homosexual relationship with her best friend" in another showcase.

on the other hand, there are numerous theaters committed to performing the classics, or musicals or Shakespeare. AMAS Repertory, for example, is shaped by the vision of its founder, actress Rosetta Le Noire, who will do only musicals with interracial casts and no profanity. The hit "Bubbling Brown Sugar" is one show that emerged from that laboratory, and Micki Grant's current "It's So Nice to Be Civilized" will be on Broadway before summer, Grant said.

The Ark Theater Co., in Soho, is the baby of actor-writer-director Donald Marcus and his actress-singer wife, Lisa Milligan, who have converted a loft a floor below the Perfect Neckband Co. into a 73-seat theater, dressing rooms, and an adjoining four-room apartment.

Their first production, an adaptation of Anton Chekhov's first play-one rarely performed because the orginal version is seven hours long-opened under a showcase contract in January. The adaptation was called "Idle Hands" and it was the best play I saw in New York. And that had nothing to do with the fact that my friend Jonathan played the lead.

It took the Marcuses 2 1/2 years from the time they decided to start their own theater, which isn't too bad in the theater world.Backed by a battery of useful friends like accountants, lawyers, an architect cousin and so forth, as well as a little family money, they scoured the neighborhoods of Manhattan looking for the "right space."

They want to do a variety of plays (the next will probably be a musical) and attract a "broader spectrum of people" than most off-off Broadway theaters. "We want to get bankers, stockbrokers, people who might not otherwise come to Soho," Marcus said. "The audience can get so ingrown."

The Manhattan Theater Club (MTC), which has an off-Broadway theater as well as a showcase and a cabaret theater in one ancient building on the upper East side, is well established enough to have a $750,000 budget ad have done an audience survey.

"Thirty-eight percent of our audience comes from between 59th and 86th Streets on the upper East Side," said managing director Barry Gove. "Our average subscriber is a $30,000-a-year professional person between 35 and 49."

The MTC, "founded in 1970 by a group of private citizens as an alternative to the commercial theater," does new plays. Their musical, "Ain't Misbehavin,' " moved to Broadway last year, won the 1978 Tony Award for Best Musical, and brought the theater $100,000 in royalties this year. They are regularly reviewed by the major newspapers, and a job there is considered a plum. A few weeks ago Patricia Elliott was starring in "Artichoke," in the larger theater, and Roberta Maxwell was in "Stevie" in the showcase space.

"I see myself as part of a network," said Bob Moss, who is sent about 3,000 scripts a year-all of which get ready by someone.Moss was instrumental in developing what has come to be known as "Theater Row," on 42nd Street between 9th and 10th Avenues.

There are at least eight off-off Broadway theaters on the Row, from Playwrights Horizons to the Intar Spanish theater, the Black Theater Alliance, the Harold Clurman Theater, the Lion Co. and the Harlem Children's Theater. They hope their presence will upgrade the rather sleazy neighborhood as well as develop an audience.

The last time I talked to my friend Jonathan, he'd gotten a job in "The Grand Tour," which stars Joel Grey. The show was going to travel to Chicago and Los Angeles and, possibly, Washington, D.C. Jonathan said his part was pretty good-he plays an aide to a colonel.

"I'm onstage a lot," he said, "I'm Polish, and I don't have many lines, but I carry a lot of luggage." CAPTION: Pictures 1 through 4, "There's more theater in New York than anywhere in the world-and it's mostly off-off-Broadway." Clockwise from upper left: Donald Marcus and Lisa Milligan outside their Ark Theater in Soho, "Theater Row" on West 42nd Street, Jonathan Hadary (center) as Herschel in "Gemini," the cast of "Vanities" outside Chelsea's Westside Theater on West 43rd Street and first rehearsal of "Ladyhouse Blues" in a vacant office building.

Photos by Donal F. Holway for The Washington Post; Picture 5, Jonathan Hadary (left) made it to Braodway in "Gemini," which first opened in Playwrights Horizons; Picture 6, Bob Moss (above) is the director of that off-off-Broadway theater. Right photo by Donal F. Holway for The Washington Post.