NEW YORK, NY.- To understand the role of the Broadway orchestrator, seek out composer Stephen Sondheim’s piano demo for the song “Losing My Mind” from the musical “Follies” and then compare it to the version on the original cast recording. The demo’s tone is wistful and resigned, with a touch of the whiskey bar about it. In the finished version, the song sounds transformed: Ascending notes on the strings, interjections from the brass and crashing cymbals build to a powerful climax, evoking the heartache and inner turmoil contained in the lyric.

What happened? The short answer: Jonathan Tunick.

“I seem to have a nose for the theater, and it’s really like that,” Tunick, prolific Broadway orchestrator, said during an interview in his book-lined study on the Upper West Side of Manhattan. “If something works, you can almost smell it.”

Sondheim himself called Tunick the “best orchestrator in the history of the theater” during a 2011 video interview with Sony Masterworks. His work can be heard in three very different Sondheim musicals on New York stages right now: “Sweeney Todd,” “Merrily We Roll Along” and Sondheim’s posthumous musical, “Here We Are.”

In fact, Tunick, 85, has orchestrated nearly every Sondheim musical since 1970, including “Company,” “A Little Night Music,” “Pacific Overtures,” “Into the Woods” and “Passion.” For other composers, he orchestrated “A Chorus Line,” “Nine,” “The Color Purple” and “A Gentleman’s Guide to Love and Murder.” An EGOT winner (that rare recipient of Emmy, Grammy, Oscar and Tony awards), Tunick won a Tony for his “Titanic” orchestrations in 1997 (the first year the award was presented) and an Academy Award for the film version of “A Little Night Music.” Last fall he became the first orchestrator to have his portrait hung at Sardi’s.

At the Sardi’s event, at least a couple of guests could be heard wondering aloud: What does a Broadway orchestrator actually do?

Typically, for a Broadway show of the kind Tunick might orchestrate, the composer provides the vocal part along with some form of accompaniment. That accompaniment can be a basic chord sheet, a fully realized piano part or anything in between. It’s the orchestrator’s task — a long and lonely one, Tunick said — to turn that accompaniment into something an orchestra can perform.

There are, of course, more poetic descriptions. In Steven Suskin’s book “The Sound of Broadway Music,” the original “Carousel” orchestrator, Don Walker, likened orchestration to “the clothing of a musical thought”; Hans Spialek, who orchestrated “On Your Toes” and numerous other Rodgers and Hart shows, compared it to “painting a musical picture.”

Tunick’s preferred analogy is “lighting for the ears.” He often confers with a show’s lighting designer to determine which colors and shadings will be used onstage. The orchestra, he said, has the ability “to provide its own shadings of light, darkness, warmth and texture to the music and lyrics.”

For the Broadway premiere of “Company” in 1970, Tunick fashioned a crisp, gleaming sound that was the aural equivalent of the chrome-and-glass set by Boris Aronson. Tunick conjured a hellacious soundscape for the macabre “Sweeney Todd”: agitated strings, blazing horns and frantic xylophones that evoke the scurrying of rats. For “Merrily We Roll Along,” he replicated the bold, brassy up-tempo sound of 1960s Broadway overtures.

Tunick sees to it that the instruments never get in the way of the words.

“He is always aware of the lyric and the dramatic moment,” said Joel Fram, music director of the Broadway revival of “Merrily We Roll Along.” He pointed to that show’s “Our Time” as an example, with its twinkling piano, simple woodwind solos, gentle rhythmic figure on the bassoon and pizzicato cello — a suitable soundtrack for the youthful optimism of the show’s protagonists at that point. “It serves the song rather than overwhelms it.”

Charlie Alterman pointed to a favorite orchestration in “Company,” for which he serves as the music director of the recent national tour.

“It’s a bubbling up of emotion somewhere inside the character of Bobby,” he said, referring to the moment in the final number, “Being Alive,” when, unexpectedly, the melody of “Someone Is Waiting” — an earlier song filled with a yearning for companionship — sneaks in like a dawning realization.

“Deep down there’s something that remembers the feeling of ‘Someone Is Waiting’ and wants to be heard,” Alterman said. The choice is intriguing on an intellectual level, “but at a gut level, it does that incredible thing that good music does, where you can’t quite explain it in your mind, but it’s clear as day in your heart.”

Tunick remembers sneaking those few notes into “Being Alive” — and that Sondheim was pleased with the addition.

“At least it showed him that I was paying attention,” Tunick said.

More than merely making the music sound pretty or palatable, a great orchestrator “is also a playwright, telling the story and reflecting character in orchestral sound,” said Michael Starobin, who orchestrated Sondheim’s “Sunday in the Park With George” and “Assassins.”

Orchestration “can hint at unspoken secrets,” Tunick said. “Things that the characters don’t say, or don’t want to say, or don’t even know.”

ONE PIECE OF MUSIC made a big impression on young Jonathan: “Tubby the Tuba,” a 1945 children’s song, centers on a forlorn tuba who longs to play the melody instead of just the bass line. Much like “Peter and the Wolf,” the song highlighted the distinct characters of the individual instruments of the orchestra.

“This idea penetrated my growing brain,” he said. “It developed into a lifelong obsession.”

Tunick had some perfunctory piano lessons as a youngster growing up in New York — “I sailed through the Diller-Quaile book in a week” — but it was a clarinet, a gift from his amateur clarinetist uncle, that kept his interest.

While a student at what is now Fiorello H. LaGuardia High School of Music & Art and Performing Arts, he started his own band and played in the school orchestra as well as in the All City High School Orchestra. He started writing music, majoring in composition at Bard College, before paying his way through Juilliard by performing with the school’s orchestra.

He was considerably more interested in what was happening at Birdland than on Broadway.

“Musicals at the time were a little stodgy,” he said. “It was disposable popular entertainment. You’d throw it out like a used Kleenex. I was a little hipper than that.”

While in college, a girlfriend introduced him to Frank Sinatra — and the possibilities of orchestral arrangement. He was struck by the way Nelson Riddle’s arrangements on Sinatra’s breakup album “In the Wee Small Hours” provided commentary, color and context.

“He was tone painting,” Tunick said.

College was followed by 10 years of fitful work as an arranger and orchestrator before a big break: orchestrating “Promises, Promises,” whose jazz-inflected score by Burt Bacharach brought a refreshingly contemporary sound to Broadway.

Emboldened by that show’s success, Tunick called Sondheim, whose originality and wit as a composer he had admired since hearing “A Funny Thing Happened on the Way to the Forum.” Tunick offered Sondheim his services for his next project.

When he first heard the piano renditions of the songs that would become “Company,” Tunick was taken aback. With a few exceptions — “Barcelona” sounds like Erik Satie by way of Brazil, he observed — the score had a sound entirely of its own.

“If anything it was sort of like Stravinsky, but not quite,” Tunick said, citing the peculiar melodies and rhythm of “The Little Things You Do Together” as an example of Sondheim’s startling originality. “What is that? In every case I had to give it careful thought.”

Initially, Tunick wasn’t overly confident in his ability to do justice to the material.

“I was terrified,” he said. But starting with “Company,” Tunick helped define the characteristic Sondheim sound. In contrast to the sumptuous blare of an entire orchestra at full blast, this was a sound defined by crisper lines, purer colors, more instrumental solos, more variation and contrast of tonal effects.

That sound is certainly present in “Here We Are,” the new musical about privileged urbanites trapped in an existential nightmare. Befitting the sinister surrealism of the source material — the Luis Buñuel films “The Discreet Charm of the Bourgeoisie” and “The Exterminating Angel” — Tunick’s underscoring at times resembles the effervescently weird music of a Looney Tunes cartoon. And, once again, the orchestra knows something the characters don’t, greeting the happy exclamation “What a perfect day!” with notes that jar and thud.

Orchestrating that show after Sondheim’s death in 2021 was “like going through the letters of a deceased friend,” Tunick said, “editing them for publication.” Tunick was happy with the result. “We went out on a high note,” he added.

The musical collaboration will carry on, though.

Having already reorchestrated several Sondheim shows — not just the ones he orchestrated originally — Tunick is adapting the score of “A Little Night Music” for full orchestra, rendering it more suitable for performance by symphony orchestras and in opera houses. He will conduct a concert and recording of the new version this year.

In an even more profound and lasting way, of course, through cast albums and successive productions, the Sondheim-Tunick collaboration will continue to inspire generations of musical theater lovers — and reward ever closer listening.

Tunick’s last meeting with Sondheim turned out to be only weeks before the composer’s death, at a concert of Tunick’s work at Sharon Playhouse in Connecticut. Tunick took the opportunity to say a few words to his longtime collaborator: “I know you hate sentimentality, but I have to tell you how much it’s meant to me working with you all these years.”

As Tunick tearily remembers it, Sondheim put his arm around him and said: “Jonathan, we’re lucky we met one another.”

This article originally appeared in The New York Times.