Mr. Bush and Cheney were meeting in a private session at the White House with all 10 members of the commission.
At the administration's request, the questioning was not being recorded, nor was a stenographer present to make a transcript. Commission members were allowed to take notes. That significantly differs with the commission's interviews with former President Clinton and former Vice President Al Gore, which a commission member said were recorded.
Mr. Bush first, and then opposed extending its life. He also initially opposed having Rice testify in public. He at first said he would only meet with the chairman and vice chairman of the panel, and only for an hour. But he relented on all counts.
The White House also resisted giving the commission access to the presidential daily briefing of August 6, 2001, in which threats of a possible attack were discussed. That was recently released publicly.
Ahead of the meeting, the White House this week said Mr. Bush had been "refreshing his memory," reports CBS News Correspondent Peter Maer.
He was preparing by reviewing documents from the months leading up to the attacks and has been consulting with White House chief of staff Andy Card, national security adviser Condoleezza Rice and White House counsel Alberto Gonzales.
Mr. Bush and Cheney are not under oath, but White House spokesman Scott McClellan said they will "tell it exactly how it happened."
McClellan said he expected Mr. Bush — not Cheney — to do most of the talking.
"You should not look at this as an adversarial process. We are all working toward the same objective here," McClellan said.
Mr. Bush plans to build his answers around the theme that he knew al Qaeda was a threat but there was no hint of the time, place or date of an attack, administration officials said.
But he faces the same challenge in the Oval Office session as he does on the campaign trail: convincing Americans that he responded appropriately to an intelligence system that CIA Director George Tenet said was "blinking red" with warnings of a terrorist strike.
There are two, sharply divided schools of thought on whether or not the president will satisfy the public.
Mr. Bush's supporters see his and Cheney's appearance as a form of closure, enabling them to show maximum cooperation and get on with other business. Critics see it as a reluctant concession after months of resistance.
Rich Bond, a former chairman of the Republican National Committee, said Mr. Bush and Cheney would "tell the whole truth and nothing but" to the panel.
"This is a closure in terms of procedures and in terms of some of the symbolism that is associated with the commission," Bond said.
Once Mr. Bush and Cheney testify, the administration will be able to say that "all interested parties" right up to the president had appeared before the panel "and no stone was left unturned," Bond said.
But Lanny Davis, who was special counsel to Mr. Clinton, said it will be hard for Mr. Bush and Cheney to demonstrate full cooperation given their past resistance to the panel and their "insisting on appearing together."
"To me, transparency is the high road the White House should have taken from day one. And it mystifies me why they haven't," Davis said. In the Monica Lewinsky investigation, Mr. Clinton answered grand jury questions and submitted written responses to questions from a Congressional committee.
Davis said Mr. Bush can only show complete good faith if he follows the lead of then-President Ford, who testified before Congress in public about his 1974 pardon of President Nixon. "History has taught a lesson that all presidents need to be transparent. Avoiding transparency on grounds of constitutional principle is not going to fly in the post Watergate era," Davis said.
John Hinshaw, a history professor at Lebanon Valley College in Pennsylvania, likens Mr. Bush's appearance before the Sept. 11 commission to President Reagan's testimony before a panel led by the late John Tower into his administration's role in the 1980s Iran-Contra affair.
Mr. Reagan is rarely remembered as the American president who sold arms to Iran to try to win the release of U.S. hostages and used the money to illegally finance right-wing Nicaraguan guerrillas, Hinshaw said.
Rather, he is regarded as the man who "stood tall" against the Soviet Union and "restored American credibility," Hinshaw said.
Since polls show that nearly four in ten Americans still believe Saddam had something to do with the Sept. 11 attacks, "that's what will color the perceptions of Bush," Hinshaw said.
Norman Ornstein, an analyst at the American Enterprise Institute, a Washington think tank, said that on balance there are "more positives than negatives" to the joint appearance by Mr. Bush and Cheney.
"Unless a president really messes up, the sense that you're being forthcoming I think helps in this case," Ornstein said. "If Bush says something really foolish, that could be a negative. But I doubt very much that that's going to happen. And the odds of that happening with Cheney there are less."