NEW YORK (NYT NEWS SERVICE ).- “This song,” director Trip Cullman said, “is killing me.”

In a windowless rehearsal room, on a sun-pecked January afternoon, Cullman and his cast were busy preparing “Unknown Soldier,” a musical that began performances at Playwrights Horizons on Friday. That song, “The Clock,” had the usual troubles: tempo, tone, choreography. But there were other problems, too — problems that partly explained why so many tissue boxes dotted the room when no one seemed to have a cold.

“Unknown Soldier,” whose plot spans most of the 20th century in 90 minutes, is about a girl who meets a boy the night before he sails for the Battle of the Somme; a grandmother rattling around a house upstate with her granddaughter; and that granddaughter, now grown, trying to piece together her family’s past. The show explores loss and how we do and don’t move on from it, and its creators have had to reckon with a loss of their own: Michael Friedman, the musical’s composer and co-lyricist, died in 2017. He was 41.

Friedman, who co-founded the documentary theater troupe The Civilians and wrote the music for “Bloody Bloody Andrew Jackson,” composed quickly and nimbly in a dizzying variety of styles: pop, rock, operetta, doo-wop, American songbook, bossa nova. His lyrics could skew conversational or literary. Sometimes he didn’t bother to rhyme; sometimes his rhymes dazzled. He was also an amateur music historian and a first-rate dramaturge, and his death created a theatrical void that was both personal — for his friends and collaborators — and professional. When he died, he left behind a variety of unfinished works and incompletely revised ones.

So now, during rehearsals for “Unknown Soldier,” lyrics like “Even dying men have some kind of memory” and “There are stories left out of history” have summoned tears. Me? I sat in the back of the room hearing the song “Penelope” — impeccably Friedmanesque in its crisp syllables — crying as quietly as I could.

“That song is rude,” said Daniel Goldstein, the show’s book writer and co-lyricist.

He and Friedman began working on “Unknown Soldier” a decade and a half ago, Goldstein told me a few days later in an interview. In 2006, the Huntington Theater Company in Boston commissioned a new musical from them. Friedman had chanced upon an article in The New York Review of Books about a World War I soldier with amnesia, which he shared with Goldstein. Inspired by A.S. Byatt’s novel “Possession” and Tom Stoppard’s play “Arcadia,” they began to write a show about present-day characters fumbling into the past.

Friedman’s score emerged — sweeping, strongly melodic, more focused than his usual pastiche style. Actor Erik Lochtefeld, who joined the show for a 2011 workshop and has remained with it since, recalled a conversation in which Friedman told him that people didn’t think he could write soaring ballads. But he did.

Mandy Greenfield, the artistic director of the Williamstown Theater Festival, described Friedman’s cello-driven score as “to my ear, his most deeply personal, sophisticated and stunning.”

She programmed “Unknown Soldier” as part of her inaugural Williamstown season in 2015. On the final day of the show’s two-week run, Friedman, Goldstein and Cullman — who had signed on as director — met for ice cream and talked about next steps. They needed to flesh out the scenes set in the past, they knew, and rework a couple of vaudeville-style numbers.

Over the next year or two, they returned to the project sporadically, trying and failing to interest other theaters. (Premieres are sexy; second productions, less so.) Then, in July 2017, Goldstein received a text from Friedman. He was in the hospital, asking for a phone charger. Within two months he died from complications of HIV/ AIDS. Until that first hospital stay, no one, including Friedman, had known he was ill.

About a year later, after helping to arrange memorials and benefits, and grappling with loss more privately, Cullman and Goldstein began to talk about “Unknown Soldier” again. “It was too sad not to,” Goldstein said. They asked Friedman’s executor — his sister, Marion Friedman Young — for permission to work on the script. She gave it.

The revisions were sometimes fraught. No one wanted to make a change Friedman would have argued against, even as Cullman and Goldstein missed the arguing.

“He was a wonderful collaborator and also an exasperating collaborator and like a total jerk about some things,” Cullman said, using a stronger word than “jerk.” They knew that Friedman would have made the show better. (He made everything better, many colleagues said.) Now they had to improve it without him, which sometimes meant jettisoning his words and cutting bars of his music.

Last October, they arranged for a weeklong workshop at Playwrights Horizons that was often raw and tearful. “Hearing lyrics through the lens of his absence was intense,” Cullman said. Lochtefeld recalled having a hard time singing “Andrew’s Story,” a meditation on loss.

“I’m still barely getting through it,” he said.

When formal rehearsals began in January, Cullman told the cast, many of whom had never met Friedman, that things might get emotional — but that he hoped they would feel joyous, too. The unofficial theme is a line spoken by Estelle Parsons, the 92-year-old actress who plays the grandmother: “Don’t wallow, dear.”

In the room, emotions have to serve the show, not overwhelm it. “It’s hard,” Lochtefeld said. “I’m not going to lie. I’m so glad we’re doing the show. I’m so glad it’s going to have a wider audience. I’m so glad we have another chance to work on it. But it’s hard.”

Cullman has been concentrating on making the story as clear as possible — not so easy when the script hops among three time periods. He knows that audience members may bring their own histories with Friedman to it, as many did when Encores! Off-Center revived his Civilians musical “Gone Missing” in 2018. “If people come to see this and use it as a way to deal with their own grief over his death,” Cullman said, “I don’t know if I can help control that.” But he knows that the show needs to stand on its own narrative feet.

“Unknown Soldier” might not be the final Friedman work to reach New York. He had other projects — like a musical version of the documentary “The King of Kong: A Fistful of Quarters” and a survey of American popular music — in various stages of completion. But for now, it is the last, best chance to celebrate his artistry. Take these lyrics that he (mostly) wrote for the show’s opening song, “The Great War”:

But I think sometimes you see a picture

Or hear a song

Or read a letter

And a person that’s forgotten comes alive for a moment

I teared up at that last line as I read it the day before a rehearsal I attended. I mentioned it to Goldstein. “Everyone picks that one out,” he told me. “But I never think of Michael being forgotten.”

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