Oscar Isaac, who stars in the Coen brothers' Inside Llewyn Davis, covered Katy Perry's Roar as a guest on Jimmy Fallon's late night show on Dec. 10.

Isaac, whose new film Inside Llewyn Davis just hit select theaters on Dec. 6 and has been receiving rave reviews from all over, came on the show to promote the movie and also sang a small surprise ditty while there. The Guatemalan-American actor is quite gifted as a musician and sang a couple of songs for the soundtrack of the film which features the likes of Marcus Mumford and Justin Timberlake.

Isaac seems to have mastered the art of playing folk music over the course of working on the film, in which he plays the role of a musician in the Sixties coffee-house folk scene of New York. It is loosely based on Dave Van Ronk's book, the Mayor of MacDougal Street.

When Fallon asked Isaac if he was always proficient in playing folk music, the actor suggested that he did play music with his high school band but he was not as familiar with playing folk, making the film his real introduction to the genre. Fallon then handed Isaac an acoustic guitar requesting he play something for the audience.

The talk show host said he could play any song, saying, "surprise us." Isaac went on to quote from the film, "if it was never new and it never gets old then it's a folk song," to introduce the song he was going to play. "You could define this song that way," he added.

The notes rang with echoing clarity as he plucked his guitar and worked a wonderful folk rendition of Perry's Roar. Soon enough, as Fallon picked up on which song Isaac was playing, he joined in too.

Inside Llewyn Davis opens wide on Dec. 20.

Watch the video here: