Music is that most ephemeral of art forms. Moments of tension and relaxation come and go at a moment. Relationships can be the same. We often hope the good parts of a relationship can last, but sometimes they do and sometimes they don’t.

“Once,” a musical that opens today at Manoa Valley Theatre, illuminates these complicated but familiar themes. It’s a touching production where music binds and propels the otherwise simple, ships-passing-in-the-night tale of romance. What’s intriguing about the production is the demands it places on the actors.

“The major twist to this show is that every single actor on the stage plays a musical instrument,” said director Mathias Maas, who is known to local audiences through his work with Honolulu Theatre for Youth. “So everything that comes out on the show is actually part of the story. The story is about music, and about love, and the love of music and that is all enacted in front of you by how these people engage with their instruments and engage into the story.

“There’s a plot of a boy meets girl, but there’s a bigger plot about how music connects people and how playing instruments is such an intimate activity.”

THE STORY of “Once” was originally told in 2007 in an award-winning indie film, winning critical praise for its uplifting message as well as its charming music. The subsequent musical, with music and lyrics by Glen Hansard and Marketa Irglova and book by Enda Walsh, debuted on Broadway in 2011 and built on that success, winning eight Tony Awards, including best musical, best book and best actor.

For Maas and music director Matthew Tadashi Mazzella, casting was the major question mark in staging “Once.” Hawaii has plenty of stage performers who can sing, dance and play an instrument. But not everyone can do all three at a performance level — after all, even on Broadway, most musicals have singing and dancing actors on stage, with the musicians a separate entity in the pit below and the musical director coordinating the two.

“It’s a hard show to do, because you need people who act and sing and play multiple instruments,” Mazzella said. “The theater was worried, like ‘How are we going to find all these people and are you sure we’re going to be able to do it?’ But then we got all the people we needed and more.’

“Usually for the auditions, you come in and give your sheet music to the pianist and you’ll sing along, and then you’ll read some scene later. Ours was different because we had no pianist, so we told everybody in the call to come with your instrument and accompany yourself — play along and sing.”

Rather than select the “best” player of each instrument, Maas and Mazzella emphasized the ensemble nature of the story, choosing the cast from those auditioners who they felt were musically the most compatible.

“I was going to take whoever was brave enough to be on stage if they can work together,” Maas said. “This show hinged on making music together.”

“Once” takes place in a “dive-pub” in Dublin, where a despondent, frustrated musician known as Guy, played by Jarren Amian, sings a song mourning his recently lost love. A strong-willed single mother, a Czech emigre known as Girl, played by Melani Carrie, is taken by his song and encourages him to embrace his talent and the beauty of his art.

They sing a song together — the Oscar-winning “Falling Slowly” — and the roller coaster ride of romance begins. “In seven days, they go through it all,” Maas said.

For Carrie, the challenge will come in playing and singing at the same time, on stage. A classically trained pianist from an early age, she has only recently started to test her vocal chops, posting some YouTube videos of her singing.

“Playing and singing, I’ve done that in front of more than just myself on rare occasions, but this is the first time I’ll have ever done it on stage,” she said. “People underestimate it. You’re using three of your limbs instead of two. When it’s rhythmically challenging, that’s when it can be difficult, but thankfully for this show the composers found the beauty in simplicity.”

An additional challenge for her is trying to speak with Girl’s Czech-accented English. The characters in “Once” are supposed to be either Irish or Czech, and MVT is maintaining the cultural milieu of the original show, even getting a Czech-language expert to coach the actors.

“That was probably the hardest element to add on,” said Carrie, who since moving to Hawaii three years ago has appeared in MVT’s production of “Allegiance” and “The Princess and the Iso Peanut.” “Trying not to sound too Russian was one of the finer points. My mother in the show, she would have been raised in the USSR times, so her accent would be much stronger than mine, as someone who has been living in Ireland. You use the back of your mouth a lot, and it’s exhausting.”

Amian fell in love with “Once” after seeing the film years ago. “My friend told me it’s like a musician’s ‘Love Story,’ so I checked it out,” he said. “It broke my heart, and then I found out there was this musical and I just jumped at it.”

He said although the storyline is the same, the musical is a different experience than the film. “You can see the passion more closely in the film, versus here we have to project the passion, because of the space.”

Amian, who would sing at high school events at Farrington, especially enjoys using the kind of gritty vocal technique called for in “Once,” recalling his early days singing.

“When I started really singing it was during the pop-punk era, so I was not learning proper technique until a little bit later,” he said. “I did learn how to work with grit in my voice and I liked singing it.”

The variety of instrumentation featured in “Once” is impressive. The production calls for guitar, piano, cello, violin, banjo, bass guitar, ukulele, mandolin, harmonica, accordion, drums, percussion and tambourine, along with the “cajon,” a boxlike drum from Peru, and the “djembe,” an hourglass-shaped drum from West Africa. Characters in the show jam together, sharing musical ideas as they draw the audience into the story.

Maas calls “Once” a musical production that “technically is not a musical.”

“There’s no point where someone is just going to leap up and burst into a song,” he said. “These are musicians. These are band people, so they’re playing music. They’re listening to tracks of music that they wrote. It’s got a slightly differently feel than a classic musical — it’s one of those shows where the music just shows up.”

“ONCE”

Presented by Manoa Valley Theatre

>> Where: 2833 East Manoa Road

>> When: Opens 7:30 p.m. today; 7:30 p.m. Thursdays, 8 p.m. Fridays, 8 p.m. Saturdays, 4 p.m. Sundays. (No show Thanksgiving Day, Nov. 28) Through Dec. 1

>> Cost: $22-$40

>> Info: 988-6131, manoavalleytheatre.com