PORTSMOUTH — Despite a several-month casting call that netted hundreds of submissions, no one really seemed right, and so the directors of Seacoast Repertory Theatre’s production of “Once” were heading toward rehearsals without a lead actor.

As a last resort, they turned to YouTube and unexpectedly found what they were looking for — in a video posted by a Phoenix guy named Sean Mullaney.

“We emailed him and he was totally interested and by the morning we had cast him and booked his flight and he was here for the first rehearsal and he is just utterly perfect,” said Brandon James, who co-directs the Seacoast production with longtime theater partner Ben Hart.

“Once” opens Friday and runs through Oct. 12 at the Seacoast Repertory Theatre, 125 Bow St., Portsmouth.

It is the story of Guy, a passionate but pained street musician in Dublin, and Girl, a Czech immigrant flower seller who plays piano. The two fall in love as they make music together. Although life gets in the way, the musical beauty they created endures.

The original movie starred Glen Hansard and Marketa Irglova, musicians who performed together in the band The Swell Season. Their chemistry during filming later grew into a romance echoing the story’s signature song, “Falling Slowly,” which they wrote and which won an Academy Award.

James and Hart scoured far and wide to cast Guy, but traditional channels proved fruitless despite reviewing hundreds of submissions.

The casting had to be absolutely perfect. Glen Hansard is not your typical hunky leading man, but the original Broadway production sort of portrayed him that way. (But) the Glen Hansard-type guitarists and artists were not showing up for the auditions,” James said.

“So we went on to YouTube, and suddenly we had thousands of guys who just sit in their bedroom with a guitar and cover the singer-songwriter tunes of Glen Hansard. We found this one guy in Phoenix that we absolutely fell in love with.”

“Once” began as a movie, blossomed into real-life romance, and was turned into a Tony award-winning musical play. The Seacoast Rep’s production opens Friday .

“It’s a universal, heart-breaking beautiful tale of two people who come together and fall in love but because of their life circumstances they can’t be together,” said James.

Playing Girl is Jackie Evans, from New York.

“It’s like the magic recipe for the show,” James said. “We saw hundreds and hundreds of really great Girls but until we knew who the Guy was we really couldn’t pull the trigger. (Evans) is a really special artist. Her piano playing is really something to behold.

In all, there are 13 actor-musicians, each playing five or six instruments.

The set features a wall of 700 multicolored bottles to evoke the rich colors of Dublin.

“We were very inspired by the colors on the streets – all the buildings are painted in this sort of muted, antique kind of jewel tones. And there’s art installations and street art and graffiti that are really vibrant in color,” James said.

It’s a cozy fit.

“We want this show to feel like a warm, autumnal sweater in a coffee shop, with a latte and a cat.”