William Finn did not set out to write different musicals, he was just "finding a voice with which I'm comfortable." But "March of the Falsettos" (now playing at the Studio Theatre), like his "In Trousers," is not made of the stuff of your average musical: Both deal with a central character named Marvin and his leaving a woman for a gay male lover.

"I always felt you could musicalize anything," says Finn, who like many in the theater grew up listening to musical comedy records. At least he thinks he did -- he professes to have a memory that retains only the preceding four days.

Now 33, the extremely shy Finn lives in New York with his piano, an upright that "scrapes the floor" and that stays in his bedroom so that he won't bother the neighbors. "A tenor and a soprano -- they've got a nerve complaining about my piano. Have you ever listened to a soprano and a tenor warming up?"

Since its origins at Playwrights' Horizons in New York, where Finn is an artist in residence ("that means when I'm low on money I call them up and say, 'Help!' "), "March of the Falsettos" has been produced at Rutgers University and in Toronto, Chicago and Los Angeles. Finn hasn't liked some of the productions, but says he would never say so. "I'm a nice Jewish boy. You can't be rude," he explains.

He sees himself as primarily a lyricist. "March" has no book at all -- the story is told entirely through the songs. It is not the subject matter that makes his work different, he says, but the language. "I'm all colloquial," he says. "They sound very untutored, unstructured. But it takes me years to produce that."

He has been compared to Stephen Sondheim, also a word man, but sees little similarity. "Sondheim is so sculpted, so precise. I prefer a longer line that seems to drift off."

Currently he is working on a musical with Charles Rubin, who is writing the book. ("I could not do what he does," Finn says.) It's called "America Kicks Up Its Heels." He's also planning a show about Siamese twins who become doctors, which opens with a song called "I Drag My Brother Around."

His day begins with a two-mile run ("I was doing five miles but I had shin splints for four months"), then he plays the piano for awhile, has lunch, makes phone calls, takes a nap, plays the piano some more, has dinner, watches TV, and plays the piano again. Gradually the work comes together into a set of theater songs, which become a show.

Next year will be the year of the musical in New York, Finn predicts. "Millions of musicals are being produced. Everyone I know is doing a musical." That doesn't bother him, as long as they're good. But most musicals today are written by "dead heads and somber pretentious little morons," he says, which is not a group he is likely to belong to.