Back at his desk, he voiced what seemed like an aside: "To my buddy who said that I'd never be the host of 'The Tonight Show' — and you know who you are — you owe me a hundred bucks, buddy."
With that, Robert De Niro burst through the curtain and plunked a hundred dollars on Fallon's desk.
But that wasn't all. In rapid succession, a parade of other celebs circled through right behind him. They included Joe Namath, Rudolph Giuliani, Lindsay Lohan, Lady Gaga, Mike Tyson, Stephen Colbert and Sarah Jessica Parker.
Joan Rivers was also among them, repaying her "debt" and making a bit of history in the bargain: 49 years earlier to the day, the veteran comedian had made her first appearance on "The Tonight Show Starring Johnny Carson" in that very studio, and had not appeared on "Tonight" since 1987, when she was banned by Carson after jumping to Fox to host her own short-lived show.
A key part of the celebratory spirit for Fallon was the fact that, after more than 40 years in Los Angeles, he and NBC have brought "Tonight" back to New York and Rockefeller Plaza, where it has reclaimed Studio 6B, once the home of Carson and, before him, "Tonight" host Jack Paar.
If viewers needed visual evidence of "Tonight's" restored New York state of mind, Fallon delivered it with flourish.
From the observation deck atop the G.E Building he occupies, he introduced U2, who, 70 stories aloft, performed a new song, "Invisible," against a magnificent New York cityscape at a perfect moment of dusk. The backdrop was so beautiful you might have sworn it was computer generated, but it was real, as was the bitter cold that had the musicians, and a legion of fans gathered round, clad in heavy winter wear.