Tell us, Robert Gates, what's the difference between working under Barack Obama and working under George W. Bush?
"That sounds like the subject of a good book," Gates said with a smile.
"It's really hard to say," he continued during an interview aired Sunday on "Meet the Press" on NBC.
"I think that probably President Obama is somewhat more analytical. And he makes sure he hears from everybody in the room on an issue. And if they don't speak up, he calls on them."
And the former president?
"President Bush was interested in hearing different points of view but didn't go out of his way to make sure everybody spoke if they hadn't spoken up before," Gates said.
Bush picked Gates to succeed Donald H. Rumsfeld as defense secretary in November 2006. The following June, Gates recommended that Bush appoint Adm. Mike Mullen as the next chairman of the Joint Chiefs of Staff, the nation's top military officer.
When Mullen was asked Sunday to compare the styles of the two presidents, he demurred.
"Well, I think individuals are always different," Mullen said on "State of the Union" on CNN. "But, you know, I mean, I wouldn't characterize them one way or the other."
That said, Mullen remarked that Obama listens to his military advisers.
"He's anxious to get the military's input to all his decisions," Mullen said. "The discussions have been broad and deep, and I've been very comfortable both with the access and the ability to give that advice."
Both Gates and Mullen remained in their positions in the opening weeks of the Obama administration. Gates told NBC that it would be "a challenge" to serve as defense secretary for the entirety of Obama's term.