<em>The Tonight Show Starring Jimmy Fallon</em> The Tonight Show Starring Jimmy Fallon

One of the best musical performances to date on The Tonight Show Starring Jimmy Fallon occurred during a commercial break Wednesday and was completely unrehearsed.

While filming promotional spots, Fallon and his guests Billy Joel and J.K. Simmons, along with the Tonight Show band, broke into a spontaneous doo-wop version of Joel's classic "For the Longest Time" - and the harmonies are pretty damn impressive. (Apparently Simmons' musical talents extend far beyond rushing and dragging in Whiplash.)

Later, on the show, Fallon and Joel did a duet together, covering the Rolling Stones' "Beast of Burden."

Check out the impromptu performance here: