When a concert grinds to a halt at interval, you do not expect the replacement keyboardist to come from row seats. But that is what happened on Saturday night at the ICC’s Darling Harbour theatre, when 21-year-old university student Sterling Nasa stepped in during La La Land in Concert and helped keep the show going.

Nasa was in the audience at the touring production, where the film is projected on to a screen while a live orchestra plays the score in synchronisation with it.

The performance ran normally until interval, which stretched to 40 minutes. Then La La Land composer and conductor Justin Hurwitz came out alone and told the audience the orchestra’s keyboardist had suddenly fallen ill. He asked if there was a pianist in the house, and one with exceptional sight-reading skills.

Speaking to Guardian Australia on Monday, Hurwitz said the extended break came with “quiet panic” behind the scenes.

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

Musicians in the orchestra called local contacts and backup offers came in from players said to be 15 to 20 minutes away, but Hurwitz decided that was too late.

“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, a University of Sydney politics and international studies student, said he hesitated before volunteering.

“I was a little bit tentative,” he 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.”

Nasa plays piano and organ and is the bagpipes tutor at his old school, Scots College. He is also a longtime admirer of Hurwitz’s work.

Once on stage, he sat at an electric keyboard and read a score he had never rehearsed. The toughest moment came during Start a Fire, the John Legend number that includes a synthesiser solo matched to the hand movements of Ryan Gosling’s character on screen.

Hurwitz said that was the section he feared most.

“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 said he had the same thought.

“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.

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

Hurwitz said the call paid off.

“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.”

Nasa’s performance carried the orchestra through the number and drew a strong ovation from the audience.

After the final bow, Hurwitz said both men were still trying to process what had happened.

“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, Nasa was moving between breakfast television and radio studios, recounting the night. He said playing music from a score he had admired for years was “quite a blessing”.

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

The production team is now rehearsing new keyboardists for the Melbourne and Brisbane legs of the tour, while Nasa returns to university lectures.

Hurwitz said the student had the talent for a music career, but the decision was his.

“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.”

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