Gunshots rang out backstage at the Academy of Music. Yet Opera Philadelphia's leading baritone, Jarrett Ott, kept talking as if nothing had happened.

"That's pretty typical for us," he explained. "Gun practice."

The opera is Jennifer Higdon's Cold Mountain, and the academy's production opens Friday. Excitement is high: A week before opening, ticket sales already had surpassed those of both Oscar and Ainadamar (new operas from previous seasons). It's an adaptation of the Charles Frazier novel about Civil War Confederate deserter W.P. Inman. Making his way home from the losing side of the war, he's so scarred he doubts he can resume any normal life with the woman he loves.

That's a lot to absorb for the 28-year-old Ott, now only two years out of the Curtis Institute of Music and having been cast in the opera's main role only days ago. "Inman would have been the same age," says Ott. "I would've been there, in those trenches in Petersburg."

But the opera - demanding from him 90 minutes of heavy singing over two-plus hours - has its own kind of casualties.

Stepping in after star baritone Nathan Gunn left due to a family illness, Ott at least has the advantage of having workshopped the opera as a Curtis student. His role is only the latest mutation of the long-in-the-works opera, which premiered last summer at Santa Fe Opera to reviews ranging from positive (the Washington Post) to skeptical (the New York Times). The piece would seem to have more distance to travel before becoming a full-fledged addition to the modern opera repertoire - and to the output of the Pulitzer Prize-winning Higdon, who has enjoyed international orchestral successes over the last 15 years since the premieres of Concerto for Orchestra and blue cathedral in Philadelphia, where she lives.

The best development scenario from Opera Philadelphia's past is Nico Muhly's Dark Sisters, which was tweaked into transformation en route from New York. Richard Danielpour's Margaret Garner, however, ran out of gas en route from Detroit.

Cold Mountain changed just by moving indoors. "It sounds a lot different," Higdon wrote in an e-mail. "Everyone who worked on it in Santa Fe has been startled when they've ...seen the set indoors."

The open-air Santa Fe Opera is one of the most beloved venues in the country. But one hears of comments about not having to compete with Mother Nature, and how singing after stage combat is easier when not at a 7,199-foot elevation. Ott isn't the only significant cast change. Jay Hunter Morris remains as the villainous Teague, and Isabel Leonard is still Ada (the woman back home struggling to survive). But Cecelia Hall replaces Emily Fons as Ruby, the tough outcast who whips Ada's farm into shape. Marietta Simpson, known for oratorio performances, makes a late-career Opera Philadelphia debut as the freed slave Lucinda.

But revision frenzy has not gripped the opera itself. The piece has remained rather unchanged from workshops three years ago at the Curtis Institute, with Ott as Inman. Higdon cut one scene, reshuffled another and added optional lower notes, giving a cue to the cast that word projection is paramount. "It's about delivering the text in the story," says Ott, "which Jennifer does so brilliantly."

This opera came to be amid a chain of coincidences. When he came to the United States 10 years ago, Opera Philadelphia's music director, Corrado Rovaris, born in Italy, chose to read Cold Mountain - in an Italian translation. Higdon hit on the book after an extensive search for an opera subject, its Southern setting being close to where she grew up in Tennessee. The area's musical vernacular is in her blood. While writing the libretto, Gene Scheer was cast as a performer in another piece of musical Americana, Rodgers and Hammerstein's Oklahoma! at the Central City (Colo.) Opera.

Numerous people at the Cold Mountain workshops singled out Ott, a native of Lehigh Valley and a graduate of West Chester University, as high-caliber vocal backup that few new operas have. When Gunn questioned his future in the production, Opera Philadelphia general director David Devan said he took it as less of a catastrophe than he might have, "knowing he had such a great cover."

Gunn's decision happened quickly. "I knew that his father was not well," said Rovaris, "but I couldn't imagine that he was going to leave. I didn't have any clue. When you see somebody working for two weeks so hard, so engaged...."

Devan scotches speculation that other matters were at play: "There are no other reasons why [Gunn is] not doing the role."

The timing wasn't great for Ott. He was already cast in Opera Philadelphia's next production, Strauss' Capriccio opening March 2. He's rehearsing both operas, the Strauss being notoriously difficult to memorize. "No pressure or anything," he says.

At least Ott has, thanks to workshops, long history with Cold Mountain. "I know every single person's lines in this opera," he said. "When you finally get up there and put the role on its feet, it's...a circus. You have to react quickly to the different thoughts Inman is having, the tempi of the music, whatever else is happening, the gunfire."

The $2.4 million production - a mass of Civil War wreckage described by one critic as giant pickup sticks - was one of the more questionable elements in Santa Fe but reportedly now has a kind of lighting design not possible amid the production traffic of Santa Fe's repertory season.

How might the opera's complexion change? "Good question," she admits. "I don't know."