As Jimmy Fallon prepared to debut as host of NBC's "Late Night," he found his predecessor, Conan O'Brien, in his dressing room, still packing up.

"I'm about to start my first show," Fallon told him eagerly. "Oh, is that tonight?" O'Brien replied. "I was gonna TiVo it, or something." A new era began Monday in the time slot commonly known as "12:30" (more accurately, 12:37 a.m. EST) as Fallon arrived with a new-era version of "Late Night," flush with his wired-up personality, a classy, comfy set and a super-cool house band, the Roots.

After the O'Brien bit, Fallon stepped on stage with a brisk, if forgettable, monologue, followed by a more clever "Slow Jam" presentation of the news (bluesy and romantic, as accompanied by the Roots). After that came a comic tribute to the show's designated target demographic: Blonde Mothers.

A possibly recurring game-show spoof, "Lick It For Ten," invited three "contestants" from the audience to lick something - a lawn mower, a photocopier and a bowl of goldfish - for a $10 prize.

And all that was before the first guest, Robert De Niro, who was game to play off of his reputation as a difficult interview.

"I wrote some questions with one-word answers," said Fallon helpfully. Consulting his list, he asked De Niro, "How are you?"

"I'm OK."

"Are you happy to be here?"

"I don't know yet," De Niro replied.

Fallon did his De Niro impression and got De Niro to do a Jimmy Fallon impression. Then they appeared in a sketch together.

Justin Timberlake came on next and plugged a new reality series with a film clip that seemed like yet another spoof, but is actually for real.

Finishing up was Van Morrison, who performed a song amid a huge band of his own.

And that's how the hour went. Whew! No one could say the pace ever dragged or the energy lagged.

It certainly didn't with its hyper host.

Fallon, a former cast member of "Saturday Night Live" with several films to his credit, has tapped a longtime "SNL" hand, Michael Shoemaker, as his producer. Co-producer is Gavin Purcell, who ran "Attack of the Show," the daily Net-centric news hour on cable's G4 channel.

Lorne Michaels (who used to be Fallon's boss on "SNL") continues as "Late Night" executive producer. So it would seem he has all the pieces in place, as he faces competition that includes Craig Ferguson on CBS' "The Late Late Show."

His major challenge in his new talk-show gig is to chill a little. Or even a lot.

"I know I'm gonna get reviewed off the first show, as opposed to the first couple of months," Fallon predicted in a recent interview. "'He's no Conan,' or 'He's no Letterman' - I just want that to be said, and put out there. Then viewers can relax and watch and enjoy."

Fallon's arrival sets in motion a carefully arranged host shift at NBC. O'Brien, who took over "Late Night" from its original host, David Letterman, in 1993, is now devoting full time to prepping his version of the Los Angeles-based "Tonight" show, where he will replace Jay Leno in June.

Then, come fall, Leno will return to the air with a new weeknight prime-time hour airing at 10 p.m.

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