It was much-anticipated, this world premiere. A new play. A major American playwright, Terrence McNally. A staging by the Philadelphia Theatre Company. A plot about opera - the same theme that had won him a Tony Award from an earlier world premiere, presented by the same company.

It opened Jan. 27 to a testy audience. The play, Golden Age, was obese, indulgent - three-plus hours long, three acts, and plodding. Who were these 19th-century characters? Why should we care? The central figure, opera composer Vincenzo Bellini, was particularly overwrought and cartoonish. The Suzanne Roberts Theatre, home of the Philadelphia Theatre Company, on some nights became noticeably emptier as the acts progressed.

In March, about three weeks after its Philadelphia run ended, the company took Golden Age, as planned, to Washington's Kennedy Center. It was shorter by 40 minutes. There were two acts, not three. Its characters defined themselves immediately. It had acquired punch, and charm. It was not just an idea, it was a realization.

It had all the same characters, played by the same actors, with scenes in the same order. McNally's basic idea was intact: By watching Bellini and his singers backstage during the opening night of his opera I Puritani in 1835, we'd get a glimpse into their psyches and their relationships with art. Yet this Golden Age was not the work that almost 6,500 people saw in Philadelphia.

What happened to Golden Age between its opening here and then at the Kennedy Center is an example of the way new American theater can evolve when an estimable playwright and a high-level theater company with access to talent and money are involved.

In Philadelphia, both parties filled that bill. McNally's many works cover a wide swath of theater, from the Tony Award-winning Master Class, which Philadelphia Theatre Company also premiered, to the wildly funny Love! Valour! Compassion! (another best-play Tony), to the books for musicals such as Kiss of the Spider Woman and Ragtime (a Tony each for McNally's scripts).

Philadelphia Theatre Company, a major player among the region's 40-plus professional stages, has a long relationship with McNally. Golden Age was its third McNally world premiere.

"I think the play has a great future," says Sara Garonzik, producing artistic director of the Philadelphia Theatre Company, who green-lighted the mountain of additional work that transformed the play in a matter of sweat-and-blood weeks - a reworking of not just the script but also the set, some costumes, the lighting and sound design, and, of course, the entire staging.

"There's more work to be done," she says, "and I'm excited about where this play could go."

For a theater company working with a major playwright, the best destination would be New York, either Off-Broadway or Broadway. Under many arrangements, the playwright would get a now-tested work with a trusted company placed on a New York stage; the originating company would garner much recognition and a small piece of the box office.

Says McNally: "It's such a long process - and I feel I'm still in the process. New plays are hard, and very brave people produce them in the current economy and environment." (Critics in Washington generally liked the play and agreed that McNally should work further to make it evolve.)

About the changes he made and the vision of a new director, Walter Bobbie, he says: "It's like the play went off to a spa and exercised and tuned up and lost some pounds and tried to get into better shape."

Even so, in Philadelphia, Golden Age's premise - that we'd gain insight by being backstage at a historic night of opera in Paris - was clearly workable, and McNally's script had sparks. He immediately began thinking about rewrites, but they'd have to be for Washington; Actors' Equity, the professional union, prohibits big changes after a production's opening, or new rehearsals while actors are performing nightly.

Another problem: The actors were to have about three weeks off, then just a few technical rehearsals - a time mostly for the production crew to adapt to the particular specs of Kennedy Center, which was to present all three McNally opera-themed plays. (Golden Age and The Lisbon Traviata have ended; Master Class runs through Sunday.)

But now, the entire cast and crew of Golden Age would need at least two weeks for rehearsals to transform the play. And that meant money.

After the Philadelphia opening, McNally and Garonzik agreed to take the play to Washington in a new form. He would rewrite the script. She would provide $45,000, the cost of the remount, a gift she'd seek from a private donor who has requested anonymity. Austin Pendleton, the director in Philadelphia who already was directing another play, would be replaced by the high-profile Bobbie (a Tony for Chicago).

That piece fell into place after the two invited Bobbie, who'd worked at Philadelphia Theatre Company and is a friend and Long Island neighbor of McNally's, to see the play in Philadelphia.

"I had a day free and I read the play before I went down and thought, 'Oh, I like this play a lot,' " he said. "Sara called and said, 'Would you think of helping us out before we go to D.C.?' "

Bobbie is not interested in play-doctoring. "I've always said no. In most cases, you sign onto a cast you never hired and a visual metaphor you never agreed upon." But when he saw the Philadelphia production, he immediately had ideas. "I felt I could help clarify what my friend had written. I felt the opportunity to help Terrence see his play."

It was a scramble, and Bobbie would have opted out if designer Santo Loquasto, also a major Broadway talent, had not figured out a way to redesign his ornate set for a play with tighter storytelling - which he did, Bobbie says admiringly, in 20 minutes. Even so, like the play, the set had the same concept in both cities.

McNally holed up in New York, rewrote - he says he took cues from Philadelphia audiences' reactions - and the new script was ready when the play closed in Philadelphia on Feb. 22. After a week off, the actors began rehearsals.

McNally says Philadelphians are open to new work and have a sense of being in on the process; in fact, one in every four plays on professional Philly stages this season is a world premiere.

"I believe the final character you add to the play is the audience," he says. "It's a privilege to work with Philadelphia Theatre Company - the audience is very smart, not cynical or jaded, but eager to listen to a story. They're also very critical. So I listen to the audience. They tell you when they're interested, restless, amused.

"Walter came in and gave a life and a vitality to the play that we can build on. And I'm very grateful to Sara. She has great faith in me and I have great faith in her. She made it possible to rework the play. These institutions have become the nurturing grounds for American theater."