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."

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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.

Williams said he's been gathering ideas for his opening monologue.

"That's my earliest chance to show some chops, to come on out and say, 'I know this is an unusual thing you're seeing.' "

Of course, world events -- and Williams' "day job" -- could unexpectedly intrude with real news.

"I reminded Lorne that it was a Saturday night that Princess Diana died," Williams said. As it happened, that week's "SNL" was a taped repeat, he recalled, "but I ended up doing a special report interrupting it.

"Lorne said, 'We'll deal with that.' "

But barring the unforeseen, Williams expects to mainly deal with butterflies.

"There are cue cards, a live audience, it's a nerve-racking atmosphere," he said. "So, sure, I'll be petrified. But I hope it's a fun, high-wire-act kind of an evening."