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“Jimmy Fallon!” WMAQ-Ch-5 anchor Allison Rosati half announced and half shouted, cupping her hands around her mouth.

And Fallon, entering the newsroom set at NBC’s Chicago station, where maybe 15 people were waiting, responded with a “Hey” that pushed the word’s syllable limit. Three, maybe? Four?

He took a look at the video wall behind the news desk, a nighttime shot of traffic streaming over the Michigan Avenue Bridge, and proclaimed, “The end of the world is here. It’s six o’clock at three in the afternoon.”

Fallon had just arrived from New York — and from a pit stop at Chicago’s Mr. Beef, where he didn’t just eat a warm, wet sandwich, he jumped behind the counter to pitch in. And he noted, not insignificantly, the name on the wall of frequent patron Jay Leno.

Fallon arrived, and the local contribution to building him into the new king of late night — presumptive host of the No. 1-rated “Tonight Show,” leaving “Late Night with Jimmy Fallon” to take over for Leno on Feb. 17, in the middle of NBC’s Winter Olympics telecasting — could begin.

The agenda, for his few-hour stay Friday, was ambitious: Promos to tape with Rosati and co-anchor Rob Stafford. Interviews with Rosati and Stafford, with the station’s morning team of Stefan Holt and Daniella Guzman, and with a reporter from the NBC Milwaukee affiliate, who had driven down.

“Tell us if we’re not doing anything right,” Stafford said.

“It’s going to be perfect,” Fallon said.

There were a couple of quick interviews with local press (one of which produced the edited Q&A; that follows). And perhaps most important, there was a little end-of-the-work-week party with the station’s advertisers, an opportunity for a car dealer or hospital executive to tell their friends forever afterward that they’ve met Jimmy Fallon, and he really is as nice a guy as he seems on the air.

And they won’t be exaggerating. Fallon, 39, has moved smoothly into the “Late Night” chair Conan O’Brien gave up to take over “Tonight” himself, briefly, in 2009. And a big part of Fallon’s appeal, both on the show and in his stand-up comedy, is his casual ease.

Even as “Late Night” has successfully tugged at the edges of the talk-show format, bringing back a lot of the just plain goofiness from early David Letterman and even “Tonight” inaugural host Steve Allen, Fallon has somehow also projected a kind of class and impish, old-world courteousness.

This is true even when, in a recent recurrence of a now-standard bit, he and tough-guy actor Jason Statham played Water War. “It’s the card game war,” Fallon explains, “but every time you lose, the one guy throws a glass of water in the other guy’s face. And we were genuinely crying-laughing. Like, I haven’t laughed that hard in a long time.”

Such a bit, Fallon says, is very much in the “Tonight” tradition.

“Steve Allen is a guy not many people have asked me about, but I feel like he’s the one that started this. I think he’d be proud of what we’re doing. I think he’d be psyched,” Fallon says.

“He did silly stuff before — like, before Letterman, he was the guy that would lay in a bowl and pretend he was a banana and you’d put ice cream and chocolate syrup in, and he’d roll around and laugh, the giant banana split. So I think if he saw our show, he’d be like, ‘Yeah, that’s the spirit.’ You want to keep it fun, keep it loose, you know, and that’s what we’re going to do. Since we started (on) ‘Late Night,’ that’s what we’ve been doing.”

In Chicago, Fallon, the former “Saturday Night Live” Weekend Update anchor, could have grumbled about the whirlwind station visits he’s been making. Last week, he taped two “Late Night” shows on Thursday, then flew to Detroit in the morning Friday and Chicago in the afternoon.

Instead, he says, “I’m making it fun … I’m making kind of like a food tour, as well.”

He entered Mr. Beef Friday afternoon without knowing Leno’s longstanding love of the place, he said. This may be true, or it may be part a bit that’ll appear on the show.

“I had just read on the Internet how good it was,” Fallon said. “You’d think someone would tell me. Anyway, here’s a photo of me outside, and I go, ‘Hey, that’s Jay’s autograph. What is Jay doing — ?’ So then I go in, and there’s a hundred photos of Jay Leno, and I go, ‘What is this? Why did no one tell me this?’ “

Looking out the windows at NBC Tower, he noted the Tribune’s historic building, the University of Chicago business school, and the Chicago River, asking, “Is the water always this green, or is it because you dye it for St. Patty’s Day? … People take a boat, even now? How fun it that?”

Fallon is professional throughout it all. He’s dressed to the nines, funny without desperation or neediness, fielding now very familiar questions about how he will change his show when it moves an hour earlier (not much), whether this “Tonight” transition is going better than the last two (oh, yes) and how he convinced NBC to move “Tonight” from Los Angeles to New York for him (he asked nicely, more or less).

When he most lit up was when he talked about his 4-month-old daughter, Winnie, the end result of years of effort by him and his wife, film producer Nancy Juvonen.

Shooting the promo spots with Rosati and Stafford, Fallon wielded a xylophone to play the three-note NBC signature — and then, in a couple of takes, at least, took a pratfall off his chair.

For the longer promo that includes his adaptation of “Late Night’s” mock-thank-you-notes segment, he asked Stafford for some intro music and then made fun of the anchor’s very busy pretend keyboard playing.

“That’s beautiful,” said Fallon. “How long have you been playing?”

The thank-you targets involved the wind and Donald Trump’s hair, delays at O’Hare International Airport, and this one: “Thank you, Chicago, for making me feel so at home — mainly because when I paid my parking meter today, it was like the down payment on a home.”

Talking in a quiet room away from the studio, Fallon shared an e-mail from Billy Crystal about a bit they’re planning together for an upcoming “Late Night,” something that is called “Schmuck Dynasty.”

“I came up with the idea of Schmuck Dynasty. I thought that would be funny,” Fallon said. “We’re both sitting there and people walk by and we go, ‘Schmuck. What a schmuck. Schmuck.’ That’s all we have so far. We’ll see how it turns out. We have a week to write it. That’s more than enough time.”

Our conversation continued from there:

Q: That kind of lo-fi stuff, big stars in lo-fi settings, works really well.

A: That’s what’s made our show work, I think. Yeah, big stars, lo-fi.

Q: So “The Tonight Show’s” been around 50-some years. How are you going to improve it and/or how are you going to not screw it up?

A: Yeah, exactly. Well, the pressure’s on. I want everyone to know that I want to honor the franchise and keep it classy and keep it the No. 1 late-night talk show. I mean this is the destination. When I was a kid, you would tune in to “The Tonight Show” before you went to sleep. Johnny Carson. A big treat. I know it’s a privilege of mine to be able to be in people’s homes. So I hope I make everyone proud, including my parents, and do a good job in this. I’ve got my nose against the grindstone. I’m working hard. I’m already working on bits for the first three weeks of the show.

Q: A concrete change I have seen you talking about is extending your monologue, and you’ve been doing this on your show already.

A: For the past three months, we’ve had 10-minute monologues (up from three or four minutes). I talked to Jay, and the one piece of advice he gave is (lengthen) the monologue. He goes, ‘because some people they don’t really get a chance to catch up on all of the news, and this is a good catch up for people.’ So at the beginning of the Tonight Show, you go, ‘Tell me. What’s up buddy? you’re the host. You’re my friend. It’s Johnny Carson, it’s Jay Leno, whoever it is. Tell me what’s going on in the world so I can function on this earth.’ You know, in 10 minutes.

Q: That’s a heavy responsibility to fill in America’s news gaps. Your current show kind of stands out, I think, for finding new ways to work with that format. It’s, you know, an old format. It’s been around.

A: You can’t reinvent the wheel. I remember when we first started out at “Late Night,” we were tyring to hire directors, and this guy was like, “I see you behind a glass desk.” I don’t. And he’s like, “Yeah, the glass desk.”

I go, “I don’t really see me as a glass desk guy.” And he goes, “Yeah, but, you know, you’re young, you should have, like, a glass desk.” And I was like, “Will you stop saying ‘glass desk’?” This guy’s meeting is not going well.

Q: “I don’t see you as our director.”

A: Exactly. (Laughs). You see that glass desk? I hope you get one. So there’s no real new way to do it. Johnny Carson gave advice to Conan back when Conan was doing “Late Night.” And he said, “It doesn’t matter what your set looks like. It all comes down to who’s sitting behind the desk.” And it’s good advice. It’s just gonna be our show. I’m not going to reinvent the wheel. Honestly, there aren’t going to be any major surprises. Basically, in our heads, we think we’re doing “The Tonight Show” now. We’re in the on-deck circle, and we’re like, “Whenever you’re ready.”

Q: Have you thought that all along, or have you sort of been gradually shifting your mindset to that?

A: I mean, I’ve just gotten older. It’s been five years, a thousand shows. I kind of know what I’m doing now. I feel really comfortable there. I’m so comfortable in my studio.

Q: So you see that spirit translating over to “The Tonight Show”? You don’t feel like you have to class it up?

A: It will just class up. I mean, we’re getting nicer furniture. The place will look nicer. I’ve always worn suits. I’ve always been respectful of my guests. I’m not going to stop that now.

Q: Are you feeling the weight of the moment?

A: Yeah, it’s getting there now. I just want it to happen, to be really honest. I just want to go. I don’t want to make a big deal about it. I know it is a big deal, but I just want to do my job and not overhype anything. I think it’ll all figure itself out. We’ll come out with the Olympics, we’ll have giant numbers. Then the Olympics will end, our numbers will go down. That’ll be a story. And then it’ll just be me, Kimmel, Dave and Conan and who’s gonna be in the lead? And we’ll be fine. We’ll be so fine. I’m not worried about this. I think it’s gonna be fun.

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