"When we shot that. We just had no ideas for what to do on the day," Efron recalls of filming the iconic number for the Disney Channel Original Movie
"That was all improv," he shared. "And we shot it, I think the whole song, in like three hours."
Related: Zac Efron Has a Sweet Reaction to Costars Ashley Tisdale and Vanessa Hudgens Being Pregnant at the Same Time
Fans of the franchise may recall that Efron's character Troy Bolton breaks out into an angry song and dance on the golf course after he has enough of everyone trying to control him -- including Sharpay Evans (Ashley Tisdale) stopping the Wildcats from performing at the country club talent show.
"When we shot that, we just had no ideas for what to do on the day," Efron explained. "The director [Kenny Ortega], he was like, 'Dude, we have no concept for this song. What do you want to do?' I was like, 'I don't know, we're on a golf course.'"
King, 24, called the dance "iconic" and shared that she was a fan of the franchise. The actress even received "a little High School Musical locker" when she was 10 years old.
Related: Zac Efron Weighs in on Those 'Really Cool' Taylor Swift, Travis Kelce High School Musical Comparisons
"I would put my pencil case in it and all this stuff, and I would press buttons and you would sing to me," King told Efron.
"No way! Oh, we have to run that back," Efron responded before giving his costar a high five.
Efron admitted that he still sings "those songs in the shower" more than 18 years after the original film premiered on the Disney Channel in January 2006.
The three High School Musical films follow a blending of worlds at the fictional East High School in Albuquerque, New Mexico, as Troy (Efron) and Gabriella (Vanessa Hudgens) contend with their friends from the basketball and theater world's disapproval of their choice to bridge the gap between the court and the stage.
"God damn it, Stanley! I do remember that being one of the most fun days that we had," Efron told PEOPLE and other audience members at the event.
Simons told PEOPLE how memorable it was working with someone he grew up watching.
"I sung [some of 'Breaking Free'] to Zac when I was playing my music scene. That was definitely a life experience that I'll never forget," he said. "When I was singing the song, he was like, 'Oh, you're not hitting it, you're not hitting it.' I was like, 'Oh,' and I redid it. And I think I did a good job!" he added.