NEW YORK — He calls it his "night off," but Brian Williams will spend it on the air as host of this week's "Saturday Night Live."

And though he'll be broadcasting from NBC's Rockefeller Plaza headquarters — the same building where he anchors "NBC Nightly News" — this assignment "is an entirely different creature," he said Thursday. "This is NOT my line of work."

A guy who's used to being in charge, he observed that doing "SNL," "You just hand it all over to these guys and say, 'You're the professionals.' It's a very interesting feeling."

And as he spoke, it was just the beginning. His first rehearsal awaited him later in the day, after his "Nightly" broadcast.

Taking on a role traditionally filled by show-biz celebrities and the occasional pro athlete or politician, the 48-year-old Williams is the first network anchor — active or retired — to host "SNL," airing tonight.

But he is no stranger to this comedy institution. He made an unannounced (and amusing) appearance on its 2006 season premiere, pretending to vie for the "Weekend Update" anchor chair vacated by Tina Fey.

For Williams, an "SNL" fan since its start three decades ago, doing the cameo was fun. Then, a few weeks ago, executive producer Lorne Michaels invited him to come back as host.

He said he gave it careful thought before agreeing.

"My family was anxious for me to do it. My bosses were anxious for me to do it," he said. "But in the end, it's ME doing it, and I worried that anyone might be confused between this and my day job. But I just don't think there's a chance of that."

He dismissed promoting "Nightly News" as a rationale for hosting the show.

"I get that a movie star goes on to pitch a movie, or a recording artist is pitching a new CD, but I don't know how many newscasts we're going to sell," he said.

As Williams spoke, lots of sketches had been written, but he had only a vague idea of what might make the cut.

"I'm going to be working very closely with Lorne on what appropriate material for me is in this show," he said.

Asked how he defined "appropriate," he paraphrased Supreme Court Justice Potter Stewart: "I'll know it when I see it."

According to Seth Meyers, "SNL" head writer and "Weekend Update" co-anchor, Williams has made just one demand: "That, when this is over, he can return to his day job with his reputation intact."

Writing for him as "SNL" host is a refreshing opportunity, Meyers said. "Actors do silly things all the time, and we ask them to do a lot of silly things in an hour-and-a-half, whereas someone like Brian is less known for his silliness. But he's pretty good with his accents. And he used to be a (volunteer) fireman: I'd bet money you'll see him on the show as a fireman."

It's also a good bet he'll be found at the "Weekend Update" desk for that parody newscast, Meyers added.