'The Voice': Watch Jimmy Fallon Prank Coaches by Wearing Disguise in Audience

Jimmy Fallon throws on a disguise to prank the coaches in a sneak peek for 'The Voice.' The 'Tonight Show' host goes on to sing Tony Bennett's classic song, 'I Left My Heart in San Francisco.' John Legend, Gwen Stefani and Niall Horan also seem seriously impressed by Jimmy's vocals, going as far to say that they would spin their chairs if he was a real contestant. 'The Voice' airs Mondays and Tuesdays on NBC.