Jack Black actually wrote the catchy ballad Bowser sings in 'The Super Mario Bros. Movie'

• Warning: There are spoilers ahead for "The Super Mario Bros. Movie."
• Bowser (voiced by Jack Black) sings a ballad, "Peaches," about his love for Princess Peach.

"The Super Mario Bros. Movie" is finally in theaters and the best part about the movie is when it lets Jack Black do his thing and deliver a hilarious unexpected singing cameo as King Bowser.

Performing on a "Ludwig von Koopa"-themed piano, Bowser belts out a ballad, "Peaches," proclaiming his undying love for Princess Peach (Anya Taylor Joy).

The chorus, which only consists of the word "Peaches" sung over and over again by Bowser, received the film's largest laughs in our crowded March 24 screening.

Fans may be surprised to learn that the song concept came from none other than Black himself.

According to the film's production notes, Bowser's musical background wasn't even in the first draft of the movie's script.

The animated film from Illumination and Nintendo follows Bowser's quest to rule the Mushroom Kingdom and make Peach his bride.

The song, a catchy banger about Bowser's desire to make Peach his, was written and produced by Black, along with directors Aaron Horvath and Michael Jelenic, Eric Osmond, and the actor's longtime collaborator and engineer, John Spiker.

"Bowser plays an incredible piano ballad that extols his love for Princess Peach," Horvath said in the press notes. "As we were trying to get the point across of Bowser's feelings for Peach, it seemed more fun, funny and sweet if we had him sing a song about it."

"Jack Black is an incredible singer and musician, so we spent a night writing the song and sent the demo to Jack," he continued. "A couple days later, Jack sent us back a finished track. He had added his own twist on it and his pianist played an original music track for it. We were blown away."

Also starring Chris Pratt, Charlie Day, Seth Rogen, and Keegan-Michael Key, "The Super Mario Bros. Movie" is in theaters now.