"It was important for me to play and sing on set because it was in the spirit of the movie to do it live," Chalamet, 28, says in the video, which features glimpses of the Oscar nominee singing ...

The crowd, realizing it was really him, cheered their inspiration and rushed to train their phones on the star as Chalamet embraced some competitors. The Associated Press even live-streamed the event.

"It was important for me to play and sing on set because it was in the spirit of the movie to do it live," Chalamet, 28, says in the video, which features glimpses of the Oscar nominee singing ...