When Sterling Nasa, 21, woke up on Saturday he could not have imagined he would soon be performing in front of 2,000 people.

On that day — which he described as “run-of-the-mill” — he helped a friend with a student film and drove around town to run some errands. Later on, he was planning to watch the movie “La La Land” accompanied by live music at the Darling Harbour Theater at a convention center in Sydney, Australia.

But by the evening, this regular day had turned into a once-in-a-life time opportunity.

“I’ve obviously hit the algorithm to some degree,” he said of his newfound internet celebrity. “I don’t think I’ve fully rationalized it yet.”

The commotion started during an interval after the first half of “La La Land,” the 2016 musical that centers on the love story of the aspiring actress Mia (Emma Stone) who meets the ambitious musician Sebastian (Ryan Gosling).

The crowd was starting to realize that something was amiss when the interval went on for longer than they expected and Justin Hurwitz, the Academy Award-winning composer of the film’s score, came onstage.

“Is anybody like an amazing sight reader?” Mr. Hurwitz asked the crowd, adding that one of the musicians had fallen ill and had to go home. For the show to go on, he needed someone to step in on the keyboard.

The request, to perform with a professional orchestra with no preparation, was nothing short of daunting. And in this case, Mr. Nasa had to flip his own pages, working his way through the show with new challenges waiting at every turn.

It was the musical equivalent of the question “is there a doctor in the house?” Mr. Nasa said.

As a clutch of hands went up at Mr. Hurwitz’s request, Scarlett Pearce — the friend with whom Mr. Nasa was attending the performance — took charge.

“I was kind of a maniac,” she said in a phone interview on Tuesday, adding that she shouted into the 2,000-person crowd that Mr. Nasa was the man for the job.

“He was just like, ‘I don’t know, maybe,’” Ms. Pearce said, as her friend tentatively raised his hand. “I grabbed his hand and put it in the air for him.”

“I don’t think he was very happy with me at the start,” Ms. Pearce, 21, said. But, she told her friend: “You’ll thank me in 10 minutes.”

After some encouragement, Mr. Nasa got up from his chair and went toward the stage where Mr. Hurwitz and the orchestra were waiting, to applause from the audience.

“I just tried not to fall down the stairs,” Mr. Nasa said.

Once he got to the stage, Mr. Hurwitz asked him for his name and handed him off to the keyboard technician who gave him the briefest of rundowns.

Half a minute later, it was showtime.

“I was pretty nervous,” Mr. Nasa said in a phone interview. But about 10 minutes into his performance, Mr. Nasa said, he found his groove and started appreciating the moment. “When else are you going to have this opportunity? You just have to go for it.”

The performance included a solo on the keyboard during the song “Start a Fire,” which is sung and performed by John Legend in the film. Mr. Nasa improvised and received loud cheers from the audience.

For Mr. Nasa, his brush with fame came at a pivotal time. He is about to graduate from the University of Sydney where he is studying international relations.

His real passion, though, is music. Mr. Nasa said his impromptu performance gave him the confidence to consider pursuing it as a career. Holding his own among such skilled musicians, he said, shows that his goals were “not completely out of reach.”