La La Land is a much adored homage to Hollywood, where dreamers take chances and seize unexpected moments.

On Saturday night at the ICC’s Darling Harbour theatre, that idea became a reality for a 21-year-old university student who was thrust into the spotlight at a live performance of the movie’s score – and saved a concert from derailment.

Sterling Nasa was in the audience at La La Land in Concert, a touring production where the movie – which features Ryan Gosling and Emma Stone – is projected on to a screen while a live orchestra plays the musical score in synchronisation with the film.

The performance proceeded normally until the interval, which stretched out to 40 minutes. Then the film’s Oscar-winning composer and conductor, Justin Hurwitz, walked out alone to address the audience.

The orchestra’s keyboardist had suddenly fallen ill. Was there by any chance a pianist in the house? And one with exceptional sight-reading skills?

Speaking to Guardian Australia on Monday, Hurwitz revealed that behind the scenes, quiet panic had set in during that extended interval.

“Our first thought was, is there a string player who also knows keyboard? The answer was no.”

As the orchestra’s musicians frantically phoned local contacts, offers started rolling in of backup players who were 15 to 20 minutes away. But Hurwitz knew time had run out.

“I figured nobody’s as close as they say they are … so I just thought, well, we have 2,500 people in here …

“Yes, it was a gamble.

“That’s why I asked a few times. I wanted to make sure that somebody wasn’t just overly confident. I asked a couple of follow-up questions like, ‘Are you sure? Can you really sight-read? Can you play key signatures you’ve never played before?’”

Nasa, who plays piano and organ and is the bagpipes tutor at his old school, Scots College, hesitated when the call went out.

“I was a little bit tentative,” the University of Sydney politics and international studies student told ABC Radio on Monday morning. “I do owe a lot of the experience to my friend, Scarlett, who sort of … put my hand up for me. But I did end up finding the confidence and it was a very good decision to go down and volunteer myself.”

A longtime admirer of Hurwitz’s work, Nasa suddenly found himself sitting at an electric keyboard, staring at a complex score he had never rehearsed.

The ultimate test came during the performance of the John Legend piece Start a Fire, which features an intricate synthesiser solo designed to match the erratic hand movements of Gosling’s character on screen. It was the exact moment Hurwitz was most nervous about.

“The synth solo is really technical, and I thought, even a really high-level professional sight-reader would probably not be able to do it,” he said. “As it was coming up, I was thinking, ‘Oh no, how’s he going to be able to handle the solo?’”

Nasa told the ABC he was thinking the same thing.

“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,” he said.

Like Gosling’s jazz-pianist character Sebastian, the student had to decide whether to stay in the shadows or take a monumental leap of faith. With no time to overthink, he chose to trust his instincts.

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

The gamble paid off, carrying the orchestra through the number – and earning Nasa a resounding ovation from the audience.

“He saw it coming up … and he just improvised,” Hurwitz said.

“That is a whole other skill on top of sight-reading. To be able to play a really cool solo in the right key, in the right scale, on the fly with no rehearsal – it was remarkable.”

The backstage debrief after the final bow was full of mutual disbelief.

“I just told him how blown away I was, and obviously how thankful I was,” Hurwitz said. “All of our heads were spinning a little bit because it was such a surreal moment.”

By Monday morning, the 21-year-old was experiencing a different kind of whirlwind, being ferried between breakfast television and radio studios to recount his sudden taste of showbiz fame.

Reflecting on the incredible turn of events, Nasa said it was an unforgettable privilege to play a soundtrack he had loved for years.

“It was quite a blessing to get to play a work that I’m in such admiration of,” he said.

While the production team is now scrambling to rehearse new keyboardists for the upcoming Melbourne and Brisbane legs of the tour, Nasa will be heading back to his regular university lectures.

But has the student missed his calling in life?

Hurwitz said that while the young Sydneysider certainly has the talent for a career in music, the choice is ultimately his to make.

“I don’t know what he’s most passionate about,” Hurwitz said. “Maybe he likes international relations a little more than music. But that’s what La La Land is about. You’ve got to do what you love the most.”