Some of the devotees of ‘Titanique,’ which recently moved to the larger Daryl Roth Theater after months of sold-out shows, have seen it more than a dozen times.On a recent Tuesday night at the Daryl Roth Theater in Union Square, temperatures outside hovered in the mid-30s, but inside, a few hundred 30-somethings in sailor hats were sipping “Iceberg” cocktails and grooving to Lizzo’s “Juice.” A gleaming silver and blue tinsel heart hung suspended above the stage like a disco ball.And then: The woman they were waiting for arrived.“It is me, Céline Dion,” said Marla Mindelle, one of the writers and stars of the “Titanic” musical parody show “Titanique,” casting aside a black garbage bag cloak to reveal a shimmering gold gown — a nod to the witch’s entrance from “Into the Woods” — and sashaying her way to the stage to a tidal wave of applause.The sold-out crowd of 270, who sported tight green sequin dresses, black leather jackets and hot pink glasses, had gathered for a special performance commemorating the 25th anniversary of the 1997 blockbuster film, set to hits from Dion’s catalog. Since opening at Asylum NYC’s 150-seat basement theater in Chelsea in June, thanks to strong word of mouth and a passionate social media following, the show has been consistently sold out.“The movie and Céline are still in the zeitgeist,” said Constantine Rousouli, who plays “Titanique”’s romantic male lead, Jack, and created the show with Mindelle and Tye Blue, who also directs.From left, Tye Blue, Constantine Rousouli, Nicholas Connell and Marla Mindelle, the creative team behind “Titanique.”Evelyn Freja for The New York TimesThe show has won praise for its campy tone, improvised moments and energetic cast, and has cultivated a fan army of “TiStaniques,” some of whom have seen the 100-minute show more than a dozen times.“It’s filled with so much joy and heart and just dumb fun,” said Ryan Bloomquist, 30, who works in Broadway marketing and has seen the show five times.The Unsinkable Celine DionThe Canadian superstar has won over fans with her octave-hopping renditions of songs like “Because You Loved Me” and “My Heart Will Go On.”Rare Disorder Diagnosis: Celine Dion announced that she had a neurological condition known as stiff person syndrome, which forced her to cancel and reschedule dates on her planned 2023 tour.Quebec’s Love Will Go On: The extraordinary outpouring in Quebec that greeted Dion’s announcement showed how her fandom, and ideas of national identity in her home province, have evolved.A Consummate Professional: At a concert in Brooklyn in 2020, the pop diva was fully in command of her glorious voice — and the crowd gathered to bask in it.Adored by Fans: Dion can count on some of the most loyal supporters in the industry. In return, she gives all of herself to them.Partially improvised and best enjoyed with a drink in hand, “Titanique,” which retells the story of “Titanic” from Dion’s perspective and through her music, began life as you might expect: during a drunken discussion between Mindelle, 38 (Broadway’s “Sister Act” and “Rodgers & Hammerstein’s Cinderella”), and Rousouli (“Wicked,” “Hairspray”), 34, at a bar in Los Angeles in 2016.Rousouli and Mindelle, a fellow “Titanic” fan, had become friends while doing dinner theater and pop parody musicals in Los Angeles. And now, Rousouli had an idea: What if they did a “Titanic” parody musical — using Dion’s songs — and made the Canadian singer herself a character in the show?He said he thought, “She’s just going to narrate the show like ‘Joseph,’” referring to the 1968 Tim Rice and Andrew Lloyd Webber musical, “Joseph and the Amazing Technicolor Dreamcoat.” (It was during this same conversation, he said, that the trash bag entrance idea in the first scene came to life.)Convinced they were onto something, Mindelle and Rousouli worked with Blue, 42, an acquaintance from the Los Angeles dinner theater circuit, to write a script. (The music supervisor Nicholas Connell, 35, did the arrangements and orchestrations.)A giant tinsel version of the blue diamond featured in the 1997 film.Evelyn Freja for The New York Times“I never considered myself a writer,” Rousouli said in a lively conversation earlier this month with Mindelle, Blue and Connell in the theater’s basement bar space. “People ask me now, ‘What was the process like?’ And it was like I closed my eyes, and all of a sudden there was draft there and I’d written this whole musical.” They wrote the initial book in a month and a half, he said.They began doing pop-up concerts of the show-in-progress at small venues around Los Angeles in 2017 and then New York the next year. The first performances were bare-bones affairs, with no set or costumes and, according to Mindelle, a “really bad” Dion accent in the first readings. But audiences loved them — and many came back for a second or third time.After a pandemic delay, they opened the first fully staged production of “Titanique” at the Asylum in June. The first month was a little scary, Blue said, with entire rows sitting empty. But by July, thanks to social media buzz, they were selling out shows. It helped that Frankie Grande, who recently had his final performance in the dual role of Jack’s pal Luigi and the Canadian actor Victor Garber, has a famous half sister, Ariana, who gave the show a shout-out after attending.“Social media and word of mouth has just been wildfire for us,” Mindelle said.Soon, celebrities were coming to see it, among them Garber, who played the shipbuilder Thomas Andrews in the film, and Lloyd Webber.“He looked at us and he goes, ‘You’re all mad,’” Rousouli said, affecting a British accent in imitation of Lloyd Webber. “I said, ‘Cool, thanks, we are.’”The production’s scrappy spirit remained when it moved to the larger Daryl Roth Theater in November, where the show now features richer sound and around 100 more seats.“I was afraid we were going to lose that sense of intimacy and charm,” Mindelle said. “But we’re now running in the audience the entire time; I can still make eye contact with people, I can still touch every person.”Members of the cast rehearsing. Unlike a typical Broadway musical, the “Titanique” script is updated weekly, sometimes daily, to stay current with pop culture references and TikTok trends.Evelyn Freja for The New York TimesPart of the appeal, said Ty Hanes, 29, a musical theater actor who has gone 13 times, is that no two performances are the same. He looks forward to seeing what Mindelle will do in the five-minute scene between Rose and Jack that she improvises every night (some of his favorites: a bit about a toenail falling off and a riff on Spam, the tinned pork product).“You can tell they just have a blast changing stuff up a bit every night,” he said.“Sometimes it really works, and sometimes it doesn’t,” Mindelle said.“No, it does,” Rousouli said. “It always lands.”Unlike a Broadway musical like “Wicked,” in which the script does not change after the show opens, Rousouli said, they tweak the show weekly — sometimes daily — to stay current on pop culture moments and TikTok trends. On a recent night, a joke featuring a Patti LuPone cardboard cutout drew loud laughs (“You can’t even be here, this is a union gig!”), and a line originally uttered by Jennifer Coolidge’s character in the Season 2 finale of the HBO satire “The White Lotus” (“These gays, they’re trying to murder me.”), now spoken by Russell Daniels performing in drag as Rose’s mother, received a mid-show standing ovation.“People feel like they’re part of something special every night,” Rousouli said.One aspect of the show’s popularity that has been rewarding, if unintentional, Mindelle said, is how L.G.B.T.Q. audiences have embraced it. “I never thought that we were writing something inherently so queer,” said Mindelle, who like Rousouli, Blue and Connell identifies as queer. “It’s just intrinsic in our DNA and our sense of humor.”Bloomquist, who is gay, said the show resonated with his personal experience. “Everything that’s coming out of the show’s mouth, you’re like ‘Oh my God, this is just how I speak with my friends,’” he said.The musical, which announced its fourth extension last week and continues to sell out a majority of its performances, is set to close May 14, but Mindelle said an even longer run may be in the cards.“I think the show has the potential to be much like the song,” she said. “We hope it will go on and on and on.” More