With a TV on mute providing cues, he and his 8-Bit Big Band were recording the music from Mario Kart 64, blown way out with 36 instruments, including saxophones, trumpets, trombones, 11 violins, three cellos and a harp.
“You got everything we need for that section?” he asked his engineer. “Great. Let’s move on to Level 3.”
At 28, Rosen is an orchestrator, composer and arranger with his hands in many musical cookie jars.
He is the music supervisor and orchestrator for Broadway’s “Be More Chill,” a show for which he also plays guitar in the pit. He has one big band devoted to video game songs and another devoted to theater music, which will perform at Feinstein’s/54 Below Monday.
Rosen also produces commercial soundtracks for the skin care company Olay out of his tiny home office in Harlem, where he stashes 52 instruments in a 9-foot-by-10-foot room that overlooks a quiet side street.
There’s a melodica, a flugelbone, a zither and a theremin, which makes the ooo-oo-ooo sound from old sci-fi movies and is played by waving your hands around in the air. If you count what he calls his “toys” — like harmonicas, tambourines and a kalimba — Rosen has 70 instruments at home. He can play them all.
“That’s insane,” he said after counting. “I’m insane.”
This collection represents more than a nifty party trick. Indeed, while he considers himself primarily a bass player, his facility with so many instruments is part of what makes him such a valuable orchestrator.
“It’s kind of like cooking,” he added. “The more spices you’re familiar with, the more you can combine them to create new flavors in places you might not expect.” Once a year, he estimates, he’ll be working on a piece of music and think, “You know what this needs? A bass melodica.”
Rosen exists as a sort of bridge between genres and generations, embracing Broadway standards, pop songs and the music of Tetris. And all of it funnels into his work.
Marc Shaiman, a Tony-winning theater and Oscar-nominated film composer, described Rosen as a big talent, but without the eccentricities that sometimes come along for the ride. “It’s the kind of talent where I almost want to hate him,” Shaiman said. “But I can’t.”
Jennifer Ashley Tepper, a producer on “Be More Chill” and the creative and programming director of Feinstein’s/54 Below, described him as a hub for young Broadway musicians, since, between his bands and the shows for which he hires, he employs so many of them.
“He’s an unbelievably calm and steadying source,” Tepper said. “When you find out that a big press person is coming, or that something has fallen apart and we can’t put it back together, he’s the first person to be like, ‘OK, let’s come up with a plan.’”
Rosen, an easy-to-smile bundle of bearded energy, has lived in the same six-bedroom Harlem apartment since he was 22, with a rotating cast of roommates. His girlfriend, Danielle Gimbal, does the copy work for his music and is a frequent presence in his home and at recording sessions.
Raised in Los Angeles, Rosen is the son of a classical bassoonist (his mother, who also plays flute, clarinet, piccolo and sax) and an organist (his father, who also plays guitar, banjo and accordion). A full-size Wurlitzer pipe organ, the kind from silent-movie houses, sat in his childhood home.
When he was 3, his parents noticed he could tell if a note played on the piano was white or black — he had perfect pitch. So he started on the piano, with his mother as his first teacher.
He did not set out to work on Broadway. But as a 15-year-old jazz student at a performing arts high school, he auditioned to play in the pit for a regional production of the musical “13” — and off he went.
He attended Berklee College of Music in Boston for four semesters, on and off between Broadway shows. He gravitated to courses he felt would help him professionally, like music production, arranging and Afro-Caribbean percussion.
His main gigs now are playing guitar for “Be More Chill” — his instrument, dressed for the occasion, wears a brown hair bow on its headstock — and co-orchestrating the production of “Moulin Rouge!” scheduled to open on Broadway in July. (On the first day of performances for “Be More Chill,” he was told by his union’s new leadership that contract rules prohibited him, as the show’s orchestrator, from playing in the band, but he was put back in while the contract is being renegotiated.)
Thanks to Rosen, “Be More Chill” features a theremin, which does its electronic ooo-oo-ooo thing when a mind-controlling supercomputer dies. There is also a flugelbone, which he described as his “orchestrating secret weapon”: It’s a horn that can be managed by a trumpet player but can sound sort of like a French horn or a trombone, which allows one musician to produce a variety of timbres for a small orchestra.
Joe Iconis, who wrote the music and lyrics for “Be More Chill,” said Rosen’s ability to have so many instruments in his head, but also so many different kinds of music, both old and new, can give his work an air of “magic.”
“His breadth of knowledge of music of all types is just intimidating,” Iconis said. “As much as he knows the synth sound that was used on the latest Ariana Grande record, he knows the model of trombone used in orchestration for ‘How to Succeed in Business Without Really Trying.’”
Rosen has lots of plans — he’d like to write music for video games, score a film, be commissioned by a major symphony orchestra to write a piece — but no particular endgame in mind, he said. He just wants to keep going.
“You don’t get to be Charlie without being insanely ambitious,” said Jason Robert Brown, who was the composer and lyricist for “13” and has worked with Rosen on several concerts and shows since. “But I think it’s really an ambition to have as much music in his life and in his head and in his mouth as he possibly can. He just loves making music.”
This article originally appeared in The New York Times.