Steve Kazee and Cristin Milioti. Photo by Joan Marcus.

This is because writers Enda Walsh (who wrote the libretto) and Glen Hansard and Markéta Irglova (who wrote the music and lyrics, and did for the movie as well), despite daring to diverge some from their source, have not made their theatre piece as thoughtful and serious as what they're adapting. The foundational relationships and plot points may be the same, but they've been so simplified and flattened for today's audiences and budget-conscious production mindsets that the considerable charm the conjured onscreen has all but evaporated.

It's easy to see the genesis for their choices. The film, made on a shoestring budget and at best informally scripted (by John Carney, with improvised additions from the actors), is full of gently prodding ideas and unsettling silences that can toe the line a bit too much between characters who don't know what to say and artists who don't know how to say it. More concrete dialogue, and firmer foundations for both the Dublin guy and the Czech girl who fall in awkward love during a brief but intense collaboration on his musical demo disc, would help crystallize the work's intense perspective on love that can't—and probably shouldn't—be.

All the seeds, however, are there, and quite moving even in their raw state. But rather than make small adjustments to Carney's ultra-lean screenplay, Walsh has manhandled and inflated it beyond the boundaries of mere clarification. By expanding and vulgarizing the characters that surround the unnamed Guy (Steve Kazee) and Girl (Cristin Milioti) at the story's center, Walsh has diluted the romance that was its entire reason for being. As a result, you lose sight of what drives them—and, worse, what threatens to drive them apart. These changes come under the questionable banners of both "concept" and "comedy."

Director John Tiffany has staged the entire show on a set (by Bob Crowley, who also designed the costumes) that doesn't just look like a pub, but is a pub: Audience members can (and are encouraged to) buy drinks there before the show and at intermission. The preshow festivities also involve various members of the cast, all of whom additionally double as the show's band (on guitars, celli, violins, accordions, and so on), singing various songs, with the street-musician Guy the last to appear. If the creators believe he is no better than a random voice in the alcohol-fueled wilderness, how can and why should we think differently?

Likewise, laughs emerge from moments of dishonest stagecraft. For example: During their first meeting, the Guy mentions he fixes vacuum cleaners, the Girl claims she has a Hoover in need of repair, and someone slides her one from offstage. Yet this violation of the theatrical reality of their still-burgeoning union only further convinces us that nothing is to be taken seriously. When Billy (Paul Whitty), the owner of the music store where the Girl routinely plays pianos because she can't afford one to buy, starts making karate-chop stances to show his protectiveness; the bank manager (Andy Taylor), from whom the two try to secure a loan to pay for studio time, croons horrific lyrics while strumming his guitar; or the girl's roommates are pressed into band service for which they're comically unqualified, it's clear Walsh's and Tiffany's goals are not to merely unlock what was missing, but to pander and condescend to audiences in pursuit of "entertainment."

The good news is that Hansard and Irglova have left their songs mostly alone, both in their content and their presentation. The movie thrived on the way diegetic performances magically emerged from, and yet related, to these people, and onstage the score retains the same basic quality. If none of the compositions quite reaches the heights of "Falling Slowly," the Guy and Girl's deceptively passionate (and Academy Award–winning) "first date" in the music store, they all contain a gritty yet gorgeous emotionalism that sounds as if it really did emerge from the urgency of the lonely streets. The expansion of themes from some songs into full dance numbers (choreographed by Steven Hoggett) spoils some of the effect, but this happens rarely.

Both Kazee and Milioti are slightly too polished and confident for their roles as cast-off lovers, though they act and sing them convincingly. (The biggest lapse: Kazee tends to drop in and out of his Irish accent, particularly when singing.) Reconfiguring the supporting characters into buffoons torpedoes the efforts of most of the rest of the cast; the exceptions are David Patrick Kelly, as the Guy's sensitive father, and David Abeles, who displays a compelling if understated bemusement as the studio owner who becomes a convert to the Guy's music (and is the only one who also seems to accompany in character).

Whether Once will attract people as readily is far from clear. Broadway producers are reportedly already circling it, and have been since its premiere at A.R.T. in Massachusetts in April, but so much about this treatment is one-dimensional it would seem to be an uncertain prospect. The germ of a fine, affecting stage show is here, but it's buried beneath mounds of the kind of show-biz schlock the original movie almost entirely shunned.

To find the truth of the story, and the best chance at both commercial and artistic success, the creative team should consider some trimming (the 85-minute movie has been bloated to two and a half hours onstage) and revisiting the simple, heartbreaking feelings that powered the film. Without such a vital return to first principles, lightning is unlikely to strike Once.

Once
Through January 1
New York Theatre Workshop, 79 East 4th Street between 2nd Avenue and Bowery
Running Time: 2 hours with one intermission
Tickets online and current Performance Schedule: TicketCentral