For example the real-life Benny Perkins was the source of some on-screen grumbling among his fellow contestants on account of his having already won the contest once before; in the musical Benny (played by Hunter Foster) is a bigoted, battle-tested specialist in psychological warfare. The switch doesn’t seem to have rattled Mr. Perkins, who attended the show with some of the other contestants during its 2012 pre-Broadway run at La Jolla Playhouse in California. He told Ms. Green at the time, “This is the best musical since ‘Paint Your Wagon.’ ”

As the relative newbie to the world of Broadway, Mr. Anastasio — who is credited with writing more than 150 songs with Phish and is famous for subverting them in concert — likens the “Hardbody” process to “parallel parking a cruise ship.” In order to do that, he said, “I knew I was going to have to keep an open mind and not be afraid to ask for help.” That aid has been more than forthcoming, he said, as he described the constant back-and-forth among the creative team (which includes the director Neil Pepe and the choreographer Sergio Trujillo) as resulting in “a cyclical dictatorship.”

This attention to the nuances of a new milieu comes as no surprise to Scott Dunn, who has conducted Mr. Anastasio and several major orchestras in evenings of symphonic arrangements of Phish material. “When the orchestras get ready to do a pop crossover thing like this, they always say, ‘Oh, great, a bunch of half notes and whole notes, yawn,’ ” Mr. Dunn said. “But these pieces are closer to ‘Dumbarton Oaks’ than ‘Moon River.’ ”

Mr. Anastasio grew up seeing musicals on Broadway and at the Bucks County Playhouse in Pennsylvania, and he said he practically wore out his cast albums of “West Side Story,” “Porgy and Bess,” “Sweeney Todd” and especially “Jesus Christ Superstar” as a teenager. One learning tool for “Hardbody” came during those repeat visits to the recent “Porgy” revival. “I noticed stuff like how the orchestra would pull back to accentuate a lyric,” he said.

That focus on the lyrics and the struggling, exhausted men and women who sing them has been humbling, he said. “If it’s not furthering the journey of the character, it doesn’t matter if it’s the greatest song in the world,” he added. “It’s going in the trash. This isn’t a concert. It’s a play, and plays have sets of rules.”

The “Hardbody” performance that had just ended was the first in which a new song had been introduced for one of the contestants and her doting husband. It was two weeks before opening night, and both Mr. Anastasio and Ms. Green lamented that the window for adding and removing material was about to close. “I keep joking that we’re probably one of the only songwriting teams who high-five anytime one of our songs gets cut,” he said. “That means we get to write a new one.”