I know, I know," Brian Williams says into his low-end Motorola. "What if that ever got out?"

He's talking at a rapid clip to his NBC colleague Tim Russert, the host of Meet the Press, about the exclusive Roosevelt Room meeting they just had with President George W. Bush. This was the plan: Williams and Russert and their network peers (Gibson, Couric, Schieffer, Stephanopoulos, and others) would get the president's perspective on the troop surge he was scheduled to announce in a few hours, and no quotes would be allowed to emerge without approval. But some doozies, like the one Williams and Russert are kibitzing about, slipped out of the president's mouth. When this happened, Williams recalls, he looked around at the ashen faces of White House aides, who quickly imposed a retroactive lockdown on that tidbit, whatever it was.

And so Williams keeps the secret, despite my needling across a two-foot-long folding table that separates me from the anchor of the NBC Nightly News—he of starchy wardrobe, stiff hair, and Dudley Do-Right air—on a northbound Amtrak about an hour later. He can talk about it with Russert and anyone else who was in the room, but no one on the outside, not even his wife, Jane, herself a savvy onetime TV news producer. "I call them 'go-to-the-graves,'" Williams says, tallying about a half-dozen he maintains for Bush alone. Williams hastens to add that today's just-between-us moment was not meant to shield the president from a trifling embarrassment, but instead to preserve the United States' options for multifront warfare.

This is not an era in which a news anchor can change the public perception of a war just by visiting it, as Williams's idol, Walter Cronkite, did in Vietnam. And if Williams chose to editorialize, Edward R. Murrow–style, right into the lens, his viewers would now have ample other channels to turn to if they disagreed. His success as the most-watched anchorman has been a study in caution, at a time when the worst thing that could happen isn't not getting the big job (where have you gone, Roger Mudd, John Roberts, Forrest Sawyer?) but getting it and then losing it (Elizabeth Vargas, Aaron Brown, Jessica Savitch). "Brian is off to a very fast start," says Tom Brokaw, his predecessor and mentor at NBC. "But it's a long curve. He's got years ahead of him."

Williams is known for his command of a 10-million-strong audience, his self-directed rise (without a college diploma) to one of the world's most competitive jobs, and a personality that, while not prone to outburst, is never afraid of confrontation. Part of his life is spent dodging resentment of his many blessings: the $10 million salary, the pied-à-terre in a new Upper East Side tower, and the restored farmhouse in Connecticut where he lives with his wife and their teenage son. (Their daughter is in her first year of college.) He's a big 47-year-old man, a notch above six foot one, slim from treadmill time, and ready to get in anyone's space with his long arms and outsize hands. His thumbnails are easily the size of quarters; even so, he manipulates his BlackBerry with astonishing ease. In conversation, he's a first-strike destabilizer even during low-stakes banter. A few drops of milk hit the table as I'm aiming for my coffee cup, and he covers the phone receiver to say, "Oh, it's gonna be like that, is it?" Later, he gets me a phone number for his old pal Brit Hume of Fox News, and asks if perhaps I might require help with the dialing.