A live concert performance of the Oscar-winning La La Land score in Sydney was minutes from being cancelled when the show''s composer turned to the audience for help. What happened next is the kind of moment that musicians, conductors, and dreamers will be telling stories about for years.

It was supposed to be a celebratory night at the Darling Harbour Theatre. La La Land — the 2016 film starring Emma Stone and Ryan Gosling — was screening on a large overhead projection while a full orchestra performed the score live beneath it. The intermission came and went, and the show stretched into a 20-minute delay, then 30, then 40. The 2,500-seat hall began to fidget.

Backstage, panic. The concert pianist had become too sick to continue. Calls were going out to colleagues across Sydney, looking for any working pianist who could read the score on the fly. None could be found in time.

A leap of faith

So Justin Hurwitz, the film''s Oscar-winning composer and the evening''s conductor, walked out alone and addressed the crowd.

"I figured nobody''s as close as they say they are," Hurwitz later told ABC Radio in Australia, "so I just thought, well, we have 2,500 people in here."

He asked the audience whether there was a trained pianist among them — specifically a strong sight reader, someone capable of playing music they had never seen before, in real time, under stage lights, in front of thousands of people.

A young man named Sterling Nasa, sitting with his friend Scarlet, hesitated. Scarlet did not. With her encouragement, his hand went up.

Hurwitz, knowing how complex the score is, asked several follow-up questions to make sure Nasa understood what he was volunteering for. Nasa — a bagpipes tutor at Sydney''s Scots College who had also studied piano and organ — admitted he had never played any of the La La Land music and had no preparation. But he was a longtime admirer of Hurwitz''s work.

To rolling applause, he walked down the aisle and took his seat at an electronic piano in the orchestra pit.

The synth solo

The show restarted with Nasa filling in. He hit his stride quickly, sight-reading at full tempo while the film rolled overhead. Then came the part everyone had been quietly dreading: a frantic synth solo that John Legend composed for one of the film''s busiest on-screen sequences, where the notes have to chase Ryan Gosling''s movements across the screen.

"I saw it on the score and I thought, oh, I don''t know if I''m going to be able to sight-read that in one go," Nasa told ABC. So he did the most musical thing possible: he improvised. He listened, he watched, and he played what fit.

"I took a little bit of a creative liberty and just decided to improvise, which I think ended up being a good choice," Nasa said.

Hurwitz, who had been holding his breath, agreed.

A spinning standing ovation

By the final bow, the 2,500-seat hall was roaring. Backstage, the conductor and the volunteer shook hands in mutual disbelief. Hurwitz admitted his head was "spinning."

"Yes, it was a gamble," he told ABC, "but one which paid off."

For Nasa, the night became something close to the dream the film itself is about — a young musician walking into a room as one thing and walking out as another, with the audience on its feet. The score that earned Hurwitz his Academy Award was, for one performance in Sydney, played in part by someone who had bought a ticket as a fan.

Why it lands so hard

Concert mishaps almost always end one way: a refund, an apology, a missed evening. This one ended with a story everyone in the room can tell forever, and with a quiet reminder that talent often hides in plain sight — sitting two rows back, holding a program, hoping someone will ask.

Sterling Nasa''s name was not on the playbill. By the end of the night, it might as well have been.