It is paying off handsomely this week. To promote his new box set, Bruce Springsteen chose Fallon's "Late Night" on NBC, not Jay Leno, David Letterman or Conan O'Brien. Tuesday's entire show is devoted to the Boss, his appearance a way of saying "thank you" for Fallon's well-received Emmy Awards opening, singing, "Born to Run" with the cast of "Glee."
The comic's musical chops are often on display, as when he impersonated Neil Young singing "Pants on the Ground" and his "history of rap" with Justin Timberlake, which quickly went viral on the Internet.
The rap history was born backstage at "Saturday Night Live," where Fallon and Timberlake were hanging out before the this year's season opener. Timberlake was to be on Fallon's show the next week, and they talked about singing some of their favorite hip-hop songs. After calling the "Late Night" writers, the idea turned into a musical trip through time from the Sugarhill Gang to Jay-Z.
Two rehearsals in a dressing room, two onstage and it was show time. It was Timberlake's idea to end the medley by walking into the audience for "Empire State of Mind," where people sang and swayed along.
Fallon has been playing guitar since age 15 and has a genuine talent, plus a lack of self-consciousness that enables him to go toe-to-toe with a professional musician and sell it, complete with dancing and familiar moves.
"You can't think about it," Fallon said. "The adrenaline takes over. I'm a big fan of reality shows, and I know you've got to complete the mission."
The Young skit was his personal favorite. Fallon came out in a wig and wide hat that shadowed his eyes so you couldn't quite tell whether it was the real Neil Young or not. He strummed an acoustic guitar and sang, in Young's high whine, the novelty song that had become a sensation on "American Idol" the night before.
Giggles bubbling up in the audience told him people were slowly catching on. "It was awesome," he said, and became even more so when he heard that that a day later Young himself, during a sound check, approached a microphone with his guitar and sang, "Pants on the Ground."
Fallon's love for music, along with the presence on his show of the Roots, the versatile hip-hop band that instantly became the hippest house band in late-night history, helps draw musicians to the show, said Jonathan Cohen, musical booker for "Late Night." Some choose to leave their own bands at home. Last Thursday, the Roots were the glue for the odd duet of a piano-playing Jeff Goldblum and rapper Biz Markie on Markie's "Just Friends."
Musical impersonations have always been a big part of Fallon's comedy and helped him get his break on "Saturday Night Live." He grew up in a musical family in New York's Hudson Valley. There is footage somewhere he hopes no one ever sees of Fallon and his sister in their mother's dresses lip-syncing "King Tut."
"I came from an Irish family that would have lots of parties," he said. "Everyone would get up and sing. Grandpa would come up and sing and everybody would cry. That's how Irish parties end."
His own musical tastes are typical of the iPod generation, a personal playlist that runs from classical to the Beatles to Jay-Z.
"It really is essential to his act to do music," said Bill Carter, author of "The War for Late Night: When Leno Went Early and Television Went Crazy."
"It's great that he picked it out as something to make him distinct."
Fallon's decision to move away from just doing things that the other late-night comedians do and use his music background has proved a smart move; his decision to hire the Roots may have been his smartest.
Beyond his own inclinations and talent, the addition of an established and still popular act sealed Fallon's ability to make music a key part of the show. Fallon remembers the look on producer Lorne Michaels' face when he heard the Roots at the first rehearsal and said, "Oh, my God. You've got a winner there."
The Roots has had memorable collaborations of their own, pushing a clearly juiced Paul Simon into a propulsive version of "Late in the Evening." They've also proven savvy contributors to the comedy, taking it to absurdist lengths when Fallon asked viewers to send in a suggestion for a song that he and the Roots would perform, and they came up with "I Love Your Elbows."
The Roots like sly, off-kilter references when welcoming guests onstage, as when they performed an advertising jingle that Elvis Costello's father had written when Costello came on the stage. The memory made Costello smile, and such moments make Fallon's work easier because it puts interview subjects in a good mood.
"I trust them," he said. "It's one of those trust exercises where when you fall back, you know someone is going to catch you. I know they will be there for me."
Fallon will not be jumping onstage to perform when Springsteen shows up this week.
"Oh, no, no, no, no," Fallon said. "I wouldn't do that. I wouldn't ruin that for the fans. There are certain things the fans would not like. I don't want to be such a ham. ... He's just going to go out and destroy and do some great rock 'n' roll, and I would never step in front of that train."