NEW YORK (AP) ― The party has already started by the time you enter the Broadway theater to see the musical “Once.’’

The Bernard B. Jacobs Theatre’s stage is filled with musicians jamming to Irish tunes in what looks like a Dublin pub. Real drinks are offered to the brave theatergoers willing to go up and mingle.

Eventually, come show time, everything sort of melts away ― those musicians are the cast ― and there’s just a guy up there strumming a guitar.

Pure, moving and inventive ― these are the foundations of the irresistible production that opened Sunday, and it is a study in how to beautifully adapt a movie to the stage. In many ways, in fact, this “Once’’ is better than the original “Once.’’

The musical follows the film’s simple love story between a Czech flower seller and an Irish street musician and vacuum cleaner repairman in Dublin. He’s pining for a lost love, she’s a lonely single mother ― can these two find love in their shared passion for music? Made for about $150,000, the film earned $20 million, thanks in part to an original score that included the sublime, 2007 Oscar-winning song, “Falling Slowly.’’

Anyone approaching the material with an eye for the stage faces serious hurdles: The film had a documentary feel even though it was fiction. It was only 85 minutes. And the chemistry between the lovers on screen ― Marketa Irglova and Glen Hansard ― wouldn’t be easy to recreate: Not only were they the songwriters, they also were one-time lovers.

Enter director John Tiffany, playwright Enda Walsh, choreographer Steven Hoggett and a talented cast of 12 led by Steve Kazee and Cristin Milioti. All are faithful to the star-crossed story without following the celluloid version shot-for-shot, and they even add depth to peripheral figures in the film. The musical was an off-Broadway hit and doesn’t suffer a bit from moving to a 1,000-seat Broadway theater.

Most of the songs from the film are here ― but not all. New tunes from Hansard and Irglova and reprises of old ones have been gracefully added to the mix by music supervisor and orchestrator Martin Lowe. Kazee plays guitar and Milioti her piano, while the other 10 performers play a variety of roles and instruments ― banjo, mandolin, violin, ukulele and accordion, among them ― sometimes dancing while performing.

Kazee adopts a convincing Irish accent and he has a great voice, especially when he strains with emotion. He’s pretty good looking, too, in just jeans, an undershirt and a vest. Milioti winningly plays the heroine like a sort of Manic Pixie Dream Girl from the Czech Republic ― “You must always say hello to the piano,’’ she tells the guy knowingly ― but pulls it off, even though her thick accent impedes her soaring voice.

Walsh has put in ingenious touches, like taking a bank officer who loans the couple the cash to record a demo tape and putting him in the backup band, together with the girl’s rowdy Czech friends and flatmates.

The owner of an instrument shop ― barely there in the film ― is given comedic life for an excellent Paul Whitty. One running joke is that both the Czech immigrants and Dublin natives are addicted to a local soap opera, a nice counterpoint to the would-be love story unspooling fitfully on stage.

One dangerous moment comes in a scene that never was in the film ― the two finally speak the unspoken.

“Isn’t this unfinished?’’ he tells her. “But we haven’t started anything,’’ she replies.

“No? Well it feels like we’ve started,’’ he says.

Part of the charm of the film was that this would-be couple never really discussed their relationship, just like life.

So it’s somewhat dangerous and a little cringe-inducing to have it telegraphed from the stage, as when the guy says: “I wrote these songs at another time for another girl but when I sing it’s for us, I think. It’s you I see in the songs.’’ But, ultimately, this is a confident, well-constructed piece. There are few special effects ― Bob Crowley’s set is just an interior of a pub decorated with mirrors ― and little beside 12 talented people playing away for more than two hours. You need nothing else.