"You really going to do this?" Jerry Seinfeld asked Jimmy Fallon at a recent charity event.
"What do you mean?" Mr. Fallon replied.
NBC had just announced the 39-year-old would succeed Jay Leno, who will leave "The Tonight Show" in February.
"You realize there's no end to this job," Mr. Seinfeld explained. "You'll just do this till you die. It's the pope job."
"The pope job." Mr. Fallon laughed, but Mr. Seinfeld kept going.
"Have you seen the pope?" the veteran comic asked about then-Pope Benedict XVI. "He can't even lift his head up. He's mumbling the prayers. No one even knows if he is saying prayers. That's what this is. This is what you want?"
It is -- which is how Mr. Fallon ended up in December on a whirlwind promotional trip to 25 cities.
And he spoke with both boyish wonder and professional ease about his new job -- as just the sixth host of "The Tonight Show."
"Someone was saying that more people have walked on the moon than hosted 'The Tonight Show,'?" Mr. Fallon said. "Me, I've done both, so that's really good."
It's all just sinking in -- this job that most people think is his dream job but never was.
"There's no way it could be a dream job because I wouldn't even dream," he said. "When I was a kid, you don't think that Johnny Carson is a job you could have."
He didn't even know what a job was when he was a kid. He just wanted to be a baseball player. When he found out what a job was, he wanted to work at IBM -- "because that's what my dad was; he was a technical writer, and he fixed computers."
So Mr. Fallon, who was born in Brooklyn and grew up in Saugerties, N.Y., attended The College of Saint Rose in Albany and became a computer-science major. But his grades were bad, so he switched to communications. After graduation, he moved to Los Angeles to work as a stand-up comedian.
He joined The Goundlings comedy troupe and got small parts in film and television before joining "Saturday Night Live" in 1998.
There, he and then-writer Tina Fey discovered their chemistry. She wrote for him before becoming head writer and later joined him on camera to co-anchor "Weekend Update."
He left "SNL" in 2004 but on his way out was tapped by producer Lorne Michaels to take over the 12:37 a.m. spot from Conan O'Brien, who had just been chosen to replace Mr. Leno five years from then.
"Lorne said, 'Look, I don't know if you're interested in hosting a talk show ...'
"And I said, 'I don't think so. I don't know what I'm going to do.'
"And Tina Fey was in the office and she said, 'You could totally do that. You're Irish, charming, you talk to everybody all the time. It will be good.' "
Mr. Fallon wasn't sure. He wanted to do movies.
"Cut to my movie's not working and Lorne calls me up five years later and says, 'Hey, remember, the talk show thing? You still into it?' "
Friends offered him all kinds of advice. Stephen Colbert told him something that Conan O'Brien had been told by Johnny Carson: "In this job, you'll use everything you ever learned in life."
"And boy, oh boy is that true," Mr. Fallon said.
"But I wish Steve Allen were still alive so he could see what we were doing with his baby, with 'The Tonight Show,' " Mr. Fallon said. "Because I think he'd be psyched."
Jack Paar made it more of a talk show. Johnny Carson did sketches and sang. Every host, Mr. Fallon said, put his mark on it.
While Mr. Leno and Mr. O'Brien scrapped over the show's debacle of 2010 -- when Mr. Leno stepped down and then took the franchise back from Mr. O'Brien -- Mr. Fallon remains neutral.
"I didn't have a dog in the fight," he said.
"Johnny Carson is the guy you look up to, because there are so many moments where he just made you laugh, he made you cry. The guy just had something."
Mr. Fallon is well aware of the legacy he has been handed.
Anyone he's itching to have as a guest?
"The queen would be fun," Mr. Fallon said. "I don't know what we could do together -- play Wii?"