On a recent afternoon in Los Angeles, a mezzo-soprano paced during an opera rehearsal before letting her sound loose. When she did, she appeared to shock herself — so much that she broke the fourth wall.“Whoa, whoa, that wasn’t my voice,” that vocalist, Tivoli Treloar, declared to her colleagues, and to an imagined audience. “I mean, I can’t sing like that!”A male voice in the cast parried with a hint of old-world courtliness: “Yet ’twas well sung, my friend!”Welcome to Kate Soper’s “The Romance of the Rose.”In addition to breaking the fourth wall, Soper’s latest work of music theater, which premieres on Saturday at Long Beach Opera, also collapses centuries, bringing its source material — a medieval French poem of the same name — into colloquial and witty collision with our understanding of opera as perhaps our most artifice-strewn art form.In Soper’s script, the mezzo, who is surprised to find herself singing (and so well!), is merely required to respond to that old-world praise with a simple “thanks.” But during the rehearsal, observed by video call, Treloar sang the word as though it were a grand encore, teasing its vowel sound into generous helpings of ornamentation.From left, Bernardo Bermudez, Tivoli Treloar and Tiffany Townsend rehearsing the opera, which was originally scheduled to premiere in 2020 but was canceled because of the pandemic.Maggie Shannon for The New York TimesWhen Treloar brought her “thanks” in for a long-delayed landing, others in the room laughed at her effective resolution of the comic-opera beat. Yet the director, James Darrah, wondered if she could stretch out the revelry even more on the next pass. A rehearsal pianist began the scene again, and Treloar indulged Darrah — this time earning an even bigger laugh.This playful moment of extended experimentation felt appropriate to both Soper’s work — her hyperverbal, zigzagging scores, filled with pools of tonal lushness as well as thickets of philosophical discourse — and the prolonged path to Saturday’s premiere.“Rose,” which has been highly anticipated for years, was mere weeks away from opening at Peak Performances, in New Jersey, when, in early 2020, the pandemic shut it down. It then languished until Darrah selected it for his first full season of programming as the artistic director of Long Beach Opera.In an interview, Soper described how “Rose” both extends and deviates from her earlier, celebrated pieces like “Here Be Sirens.” For one, those works, unlike this opera, didn’t have an intermission. “You do the whole thing in one fell swoop,” she said. “Whatever those Aristotelian time-place things are; it’s kind of a big gulp. For this one, the idea of a full two-act opera was interesting to me.”A rehearsal for “Rose,” in which, Soper said, “Who are these characters who think their normal language is operatic singing? What falls apart for them when they start to question that?”Maggie Shannon for The New York TimesIn “Sirens,” Soper gave subjective voice to characters who were mere devices in Homer; in “Rose,” she again reinterprets vintage literary concepts, but with expanded ambition and scope. In the three years since its canceled premiere, Soper has worked to refine the libretto. The text, she said, was difficult to write, when creating a “really strange story that was inspired by this incredibly bizarre medieval text.”In the medieval poem, the male protagonist — the Lover — is a dreamer whose affections are aimed at the symbolic entity of a rose. When his advance toward the rose is blocked, he’s schooled on and nudged toward the right way to think about love by a wide range of allegorical characters, such as Reason, Idleness or the God of Love.Two different contributors, separated by decades, worked on the poem as it is known today. And now Soper is having her turn to augment the text. Here, a figure she calls the Dreamer initially puts the character of the Lover through the various allegorical paces. (In the original poem, the pursuer of the rose is himself a dreamer.) And in Soper’s version, there’s a mysterious yet evident rapport between the Dreamer and the Lover — even as the latter, the mezzo-soprano, is still discovering her voice within this dreamy opera world.“Part of it is about: What is music, what does it mean when you sing opera?” Soper said. “Who are these characters who think their normal language is operatic singing? What falls apart for them when they start to question that?”Christopher Rountree, left, the production’s conductor, with Soper at a rehearsal.Maggie Shannon for The New York TimesAs a composer, Soper answers such destabilizing questions with a wealth of sonic reference points. She’s firmly in the contemporary classical mold, which means comfort with experimentation and extended techniques — as well as the electronic processing of acoustic sounds. But as the piece progresses, she feasts on polystylism and hummable melody.The production’s conductor, Christopher Rountree, said that, during a recent rehearsal, he had the experience of feeling “like we were solidly inside of Philip Glass.” Then, “within a second, we were in Gilbert and Sullivan,” he added. “And then, a second later, we were in a very heartfelt new-music ballad, but with a character who had not sung in that new-music straight tone yet.”“It’s amazing to see all the things that are being asked of the singers by Kate,” Rountree said. “And it’s cool that we have folks who are willing to go there.”The casting intentionally brings together vocalists from different backgrounds. The dramatic soprano Tiffany Townsend, who plays Idleness, is in the young artist program at Los Angeles Opera, where she has specialized in the standard repertoire. But, she said, she enjoys the way Soper braids different traditions together. Referring to the “Torch Song,” which is sung by Pleasure and Idleness, she said, “The harmony speaks to medieval music; but the way it’s set gives a jazz feel.”The vocalist Lucas Steele, who starred in the musical “Natasha, Pierre & the Great Comet of 1812” on Broadway, is making his opera debut in the role of the Dreamer. He said that in Soper’s writing, he sees “a height to the language that reminds me a little bit of Shakespeare” — but also a Brechtian sense of “talking to the audience, shifting in and out of the narrative.” (The rehearsal videos that Long Beach Opera has posted online give a sense of what Steele describes: Soper’s fluid approach to allegory and audience acknowledgment.)“Because Kate is so great at when she decides to insert accessible melody into a piece,” Steele said, “I think it’s going to give the audience something to grab onto, in the moments where it may start to become a little more on the experimental side of things.”Darrah said that “at an intellectual level, I look at it and I go, Oh, she’s very aware of opera as this centuries-old art form. There’s a way that she’s referencing clichés and mocking them at times, but also using the structure.”He paused for a beat, then added, “No one’s writing music like this.”Soper hopes to record the score soon. But for now, she’s enjoying the fruition of a yearslong effort that has pushed her into new creative directions. During her training as a composer — first at Rice University, in Texas, then at Columbia, in New York — she viewed opera singers as “a different species of beautiful people, swanning around wearing scarves. And I was with the composers trying to get people to play our music. I have this distant sense from it.”“I think opera for me,” Soper said, “is a premise and an element of the story rather than an actual medium that I’m writing in.”But with “Rose,” she said, she’s finding a way to edge closer to opera’s mainstream, even as she keeps questioning it on a fundamental level.“Playing around with quote-unquote real opera singers, and lower voices, and coloratura — if something about this is more of a real opera,” she said, “at the same time I can still kind of investigate what it means to be a real opera.” More