Jazz pianist and “Jelly’s Last Jam” star Mark G. Meadows poses for a portrait on historic U Street in Washington. (Jahi Chikwendiu/The Washington Post)

Mark G. Meadows is demonstrating the formal bow that he’d imagined actors take at the end of a performance. He slips one arm behind his back and the other across his waist and leans forward stiffly, as if greeting his dancing partner at a fancy ball in, say, the 1880s.

It’s funny to watch, and he knows it, but he’s had to learn better, and quickly. Meadows is starring in a musical at Signature Theatre, “Jelly’s Last Jam,” a first for him in every way. The curtain-call thing — like most of the conventions of acting — was completely foreign to this up-and-coming jazz pianist and composer. He’s hardly ever even attended a musical, let alone headlined one.

“Matt would say to me, ‘Go upstage,’ ” the good-humored Meadows says of his director, Matthew Gardiner, during a break before the final dress rehearsal of the show, in which he plays the racially and emotionally conflicted 20th-century jazz great Jelly Roll Morton. “And I didn’t know where that was. I didn’t know what all the terms mean.”

To any newcomer, it’s indeed directionally counterintuitive, the way theater people refer to the back of the stage as “upstage.” And “newcomer” is most certainly an apt description of this 27-year-old singer-composer, born in Washington, raised in a Dallas suburb and now living again in the District. It was Gardiner’s brainstorm to cast a contemporary jazz pianist as a legendary jazz pianist, when the lead role in the rarely revived 1992 musical about the life of Morton was devised for an actor who dances tap. The part was created by the late actor-dancer-singer Gregory Hines, who won a Tony for his performance.

Gardiner, the company’s associate artistic director, had an idea for a new way to do “Jelly’s,” with a book by its Broadway director, George C. Wolfe, and featuring Morton’s own music.

“I’ve been saying to Eric for six years that this is a show we should be doing,” the director says, referring to his boss, artistic director Eric Schaeffer. “It’s an important story to tell, and it’s a musical that’s never done. And I told him that I really wanted to do it with a jazz pianist, and find a different way to incorporate the tap.” (The Jelly Roll character doesn’t tap in this production, but five dancing members of the ensemble do.)

Mark G. Meadows (center) with the “Jelly’s Last Jam” ensemble. (Margot Schulman)

Onto the trim Meadows’s inexperienced shoulders, Gardiner and Signature have hoisted this big project, and so the investment in a young man of enormous promise — he’s released two albums and will make a debut in the fall at Jazz at Lincoln Center in New York — is huge. To guide him, he not only has Gardiner and choreographer Jared Grimes but also a heavy-duty supporting cast that includes Nova Y. Payton, as well as Broadway veterans Felicia Boswell (“Memphis”) and Cleavant Derricks, the last of whom you may recall as a Tony winner for his portrayal of Jimmy Early in the original “Dreamgirls.”

“I’m thinking, ‘They’re going to get some young great tap dancer,’ ’’ Derricks recalls of his reaction after being offered the role of enigmatic Chimney Man, who compels Morton to reflect on his troubled life. “So then I hear they’re going to do this with this young jazz musician? So how are they going to do this?”

It is a question that came to mind to Meadows himself last winter, after Gardiner ran across a video of the musician online and emailed him in Doha, Qatar, where Meadows was performing in a Jazz at Lincoln Center satellite club. “I’m a jazz musician!” Meadows remembers thinking. “I was too busy and too focused on my new album.” He emailed his thanks for the offer to audition, but the answer was no.

Meadows had only fairly recently set his trajectory as a jazz composer and performer. Growing up in Richardson, Tex., with his father, Gabe Meadows, himself a jazz musician — he’d visit his mother, Rosa Reeder, in Washington, where she remained after his parents broke up. Meadows played football and basketball, studied classical piano and gravitated to jazz. After graduating from Johns Hopkins University with a psychology degree, he followed up at the Peabody Institute, the school’s musical conservatory. A further turning point occurred in 2014, when he was accepted into the artist-in-residence program at Strathmore, which each year immerses six musicians, ages 16 to 32, in both performance and the business of cultivating a career. Meadows, who also teaches at the Duke Ellington School of the Arts, is building on the following he is developing, playing in and around Baltimore and Washington.

This is where the small-village aspect of a performing arts community kicked in. It turned out Payton had known Meadows since childhood — their families worshiped at St. Augustine Catholic Church on V Street NW. “He’s like my little brother,” she said to Gardiner. “I remember the day he was born!” Gardiner, meanwhile, knew Betty Scott, a retired elementary school music teacher who ran the residency program at Strathmore. Along with Georgina Javor, who became acquainted with Meadows while working at the Bethesda arts complex and was now assistant director of programming at Jazz at Lincoln Center. Scott was both a fan of and mentor to Meadows.

And so the education of Mark G. Meadows continued, on a more accelerated basis.

“Basically, I said, ‘Why can’t you do this? It’s a great opportunity,’ ” Javor, who supported his getting the Doha gig, recalls of their text exchanges. “Just do this and you’ll see that it’s one of the best things you’ve ever done.” How, one wonders, could she have been so certain?

“First and foremost he’s a tremendous pianist,” she says. “And in general, he has a very intrepid, can-do attitude. I don’t ever see him being afraid of anything. And when he performs, he radiates an energy and a joy that just comes naturally to him.” With Payton urging him on separately, Javor and Scott helped him over the first hurdle. He took the leap.

Having gone through the rigors of musical-theater rehearsals, leading up to the start of preview performances Aug. 2, Meadows laughs now at how right he was about how little he knew. Jelly Roll Morton, who is depicted in the musical as a gifted artist unable to come to terms with his heritage, is a complex character for anyone to tackle. But if one of Morton’s difficulties was figuring out to which world he belonged, Meadows says that he, too, having been born in a black urban neighborhood in the Northeast and spending his school years in a white Southern suburb, understands something about the paradoxes of identity.

“I’m a fusion of everything in my life,” he says, invoking a word that’s sometimes also used to describe the influences on his music: jazz, pop, blues. “White, black, gospel, jazz, rich, poor, middle class . . . ”

Acting, however, was not in his wheelhouse, even if he did for a lark once upon a time play Herr Schultz in a high school production of “Cabaret.” To inspire the budding musical-theater performer in him, Scott came up with (as teachers often do) a field trip. She invited Gardiner and Meadows to accompany her to the Library of Congress, where they listened to recordings of interviews with Morton, talking about his life.

At Signature, Derricks offered Meadows bits of wisdom. “What I tried to give Mark is confidence,” he says. “I’d tell him, ‘Remember who you are, where you come from, and where you’re going. And in any piece that you do, always listen to your fellow actors on the stage.’ ”

For Gardiner, the process of helping Meadows find his theater core has been intense. “I keep saying to my collaborators, Jared and Darius [Smith, the music director]: ‘I’m so zoned in on him. I have to be here with him, I have to help guide him.’ ” Day by day, he says, progress is being made. And Meadows, who began as a skeptic, now can’t get enough of the experience.

“I told Matt the other day, I don’t think I’ve ever spent this many consecutive hours thinking about anything,” he says. “I feel like I’m doing this the right way. I’m doing it the honest way: going into the theater world, and putting my own feel on it.”

Jelly’s Last Jam, book by George C. Wolfe, music by Jelly Roll Morton, lyrics by Susan Birkenhead. Directed by Matthew Gardiner. Tickets, $40-$103. Through Sept. 11 at Signature Theatre, 4200 Campbell Ave., Arlington. Visit sigtheatre.org or call 703-820-9771.