Billy Joel, J.K. Simmons, Jimmy Fallon Belt Out Doo-Wop Version of ‘The Longest Time’ (Video)

The spontaneous performance occurs while filming promos for Wednesday’s “Tonight Show”

A lot of scripted skits are meant to look unplanned on “The Tonight Show,” but Wednesday’s surprise musical number appears to be totally spur of the moment.

Billy Joel, J.K. Simmons and Jimmy Fallon gave an unrehearsed and spontaneous doo-wop performance of “The Longest Time” with Billy’s band while filming promos for the show, and the resulting video on YouTube has already gone viral.

The performance seems to be genuinely unrehearsed as Fallon, Simmons, Joel and his band take a second to figure out the key, starting note and harmonies before getting into the swing of the song.

Despite the lack of preparation, they pull off Joel’s 1983 hit almost flawlessly thanks to the talented group.

Fallon has shown a flair for singing since taking over the NBC late-night show, “Whiplash” star Simmons has a music degree and has sung in numerous musicals, while Joel is a six-time Grammy Award winner who has been nominated 23 times.

Earlier in the night, Joel, Fallon and The Roots sang a lively cover of The Rolling Stones’ “Beast of Burden” — complete with piano-straddling by the host.

“The Tonight Show” airs at 11:35 p.m. on NBC.