NEW YORK (AP) — Ryan Miller, lead singer of alternative rock band Guster, has a new gig that even he’s a little surprised about: Musical theater songwriter. He laughs that he’s gone all Broadway.

“I just went full-into the ballpit — just, like, face-first, take-me-wherever-this-is-going,” he said before a recent rehearsal of his bright new musical “Safety Not Guaranteed” at Brooklyn Academy of Music.

Miller is part of a growing trend of alt-rockers bringing a new sound to the space carved out by giants like Stephen Sondheim and Andrew Lloyd Webber.

The Avett Brothers are about to have “Swept Away” land on Broadway, Florence + The Machine frontwoman Florence Welch is working on a musical about “The Great Gatsby” and Jack Antonoff is scoring “Romeo & Juliet.” Radiohead’s Thom Yorke is laboring on “Hamlet Hail to the Thief,” a mashup of Shakespeare’s “Hamlet” and his band’s music.

Arcade Fire’s Will Butler is fresh off the success of the Tony-winning play “Stereophonic,” Sufjan Stevens songs found a place on Broadway last season with “Illinoise” and the folk-rock band Jamestown Revival is fueling the reigning best new musical, “The Outsiders.”

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“There seems to be a wave and we’re kind of catching it at a good time,” says Miller. “It felt like there was already some soil that was being laid that I wouldn’t have to break down.”

Tony Award-winning producer John Johnson, who is helping create that wave by backing the musicals “Swept Away,” “Stereophonic” and “Safety Not Guaranteed,” says the change reflects the rise of tastes of Millennials and Gen-X.

A changing theater audience

If, before the pandemic, the standard theatergoer was a suburban woman in her 60s, she’s being replaced by folks in their 30s to 50s from chic neighborhoods in Manhattan and Brooklyn, as well as bedroom communities in New Jersey like Maplewood and South Orange.

“We’re kind of following the path of the generational shift in the audience that’s happening right now,” Johnson says, adding that these new theater-goers “are open to new stories, new voices, as well as the voices that they grew up with.”

Alt-rock has been heard on Broadway before, of course, with shows like Duncan Sheik’s “Spring Awakening,” and Alanis Morissette’s “Jagged Little Pill,” but rare is a cluster like theater is enjoying now.

Scott Avett, lead vocalist and multi-instrumentalist for the Avett Brothers, has watched as his 2004 album “Mignonette” — based on a book about men adrift for days in the middle of the ocean — finds a new life on stage.

“It’s surreal, for sure,” he says. “The blooming of the story with the songs and the concept has been very surreal,” adding: “It feels like quite flattering and affirming.”

The songs have been wedded to a story by John Logan and the Avetts wrote one song specifically for the show, “Lord Lay Your Hand On My Shoulder.”

The leap to Broadway, in one way, isn’t that strange for the Americana band, who have always written narrative-driven and character-based songs.

“Show tunes are like the longing coming from somebody that then is exaggerated to the stage,” says Avett. “We grew up projecting that from a smaller place.”

The path of ‘Safety Not Guaranteed’

If “Swept Away” began as a book that became an album, “Safety Not Guaranteed” started life as a 2012 sci-fi comedy starring Aubrey Plaza, who played one of a trio of journalists hunting whoever placed an intriguing classified ad: “Wanted: Someone to go back in time with me. This is not a joke,” it reads. “Must bring your own weapons. Safety not guaranteed.”

Aside from the time-travel silliness, the film and musical is about the fading dreams of aspiring professionals in their 20s and 30s who see diminished opportunities and want to go back to the comfort of the past.

Miller initially had no intentions of making musical theater. When not touring with his four-piece band, he scores films, appears on podcasts and even starred in his own PBS series. Broadway or Broadway-adjacent didn’t appeal.

Miller, who lives in Vermont, was simply turned off by the sheer amount of time it usually took: Fellow Vermont creators Anaïs Mitchell’s “Hadestown” took 14 years and Trey Anastasio’s “Hands on a Hardbody” took 10 years and closed after less than a month.

But Miller found himself gradually sucked in and enjoying a timeline that was accelerated. He was originally asked just to attend a workshop performance of “Safety Not Guaranteed” studded with Guster songs — basically, a Guster jukebox — and loved it. “Have I just been like a closeted theater kid my whole life?” he asked himself.

Soon he was writing original songs for it. He was a natural, since he had scored the film, knew the characters and the emotional beats. He wrote five new stage songs in a month and sent them to book writer Nick Blaemire.

“I remember after I finished that fifth song, I sent it to Nick, I was like, ‘Man, really fell in love with this.’ Like something happened and I’m going to be really bummed if I don’t get to just see it once.’”

Miller then went on a writing tear, offering some 15 new songs in place of the Guster ones. “This was the rule: I was like, ‘Just let me beat it. If I don’t beat it, we’ll all know and that’s it.’”

One new song — “I Wanna Go Back” — shows Miller clearly having fun, with the lyrics: “We’re losing our hair/It doesn’t seem fair/All the best of our days are behind us and gone/Xbox/cheap buzz/tan lines/peach fuzz.”

Only three Guster songs — including “One Man Wrecking Machine” — made the cut. The rest are from Miller, who relished the storytellers’ task of creating songs specifically tailored for the characters and moments.

“It’s been like, one of the greatest thrills, I think, of my artistic career to have been brought/stumbled/actively weaseled my way into this world. And I’m totally smitten,” he says.

More and more alt-rockers may follow, attracted by the chance to show their multi-hyphenate-ness and the constancy. No matter how successful a rock band is, life on the road means one or two nights in a city; a stage show, on the other hand, is more like a residency.

Miller says he’s also embraced the narrowness of the role. When he’s writing for Guster, the song can be about anything. When he wrote for “Safety Not Guaranteed,” the songs had clear guidelines.

“Having limitations actually really is a very compelling, very inspirational way because it can’t just be everything. It’s got to be within,” he says. “I call them sandboxes. I love a sandbox to play in and I love limitations.”