When Mr. Lauer pressed him about his more critical decisions in office, Mr. Bush was both forthcoming and maddeningly opaque. He scoffed at the notion that he cared that critics viewed him as a puppet manipulated by his strong-willed vice president, Dick Cheney, then kept underscoring ways he had defied his No. 2, including by refusing to grant Mr. Cheney’s chief of staff, I. Lewis Libby Jr., a pardon after Mr. Libby was convicted of lying during the investigation into the unmasking of a C.I.A. operative. (Instead, Mr. Bush commuted his sentence.)

Mr. Bush ruefully acknowledged that he mishandled the Hurricane Katrina crisis, even writing a new script for his first trip to the disaster area. “I should have touched down in Baton Rouge, met with the governor, and, you know, walked out and said, ‘I hear you,’ ” he said. “And then got back on a flight up to Washington. I did not do that. And paid a price for it.”

But he refused to apply hindsight to the invasion of Iraq. Mr. Lauer asked him whether he would do the same thing again if he had known then what he knows now. Mr. Bush replied: “I, first of all, didn’t have that luxury. You just don’t have the luxury when you’re president.”

He added, “I will say definitely the world is better off without Saddam Hussein in power, as are 25 million people who now have a chance to live in freedom.”

It was a curious form of hedging from a president who prides himself on plain speaking.

There were other moments when Mr. Bush was defensive, but for the most part he looked more relaxed and unguarded than that at any time in his presidency, even chuckling about history’s final verdict. (“I hope I’m judged a success, but I’m going to be dead, Matt, when they finally figure it out.”) Throughout his tumultuous two terms, Mr. Bush rarely looked comfortable in formal interviews and news conferences.

This new book-tour persona harks back to his days as a presidential candidate, before the attacks of Sept. 11 and other crises curbed his breezy confidence and stiffened his demeanor.

Mr. Bush, who in the coming days will sit down with Oprah Winfrey and many other celebrity interviewers, says he wrote his book to give readers — and future historians — a sense of what his days in the White House were really like. Mostly, however, the NBC interview offered viewers a visceral reminder of what Mr. Bush was like before he entered the Oval Office.