In a democratic society, what is the point of questioning a candidate or any other powerful figure? When the network gets its “get,” what does the public get?

The purpose isn’t self-evident. The host of a Sunday morning show in particular—America’s bulliest pulpit—can’t justify him- or herself by serving as a “stenographer with amnesia,” as the late Jack Newfield said once, and memorably, about the general run of Washington reporting. The network higher-ups certainly crave a bump in eyeballs and eardrums, but that’s a private interest and cannot be the whole of the public’s due, either. The candidate may enjoy his or her minutes in the national spotlight, but the exercise is not intended for their pleasure.

To perform a public service, an interview ought to (in journalists’ jargon) “advance the story”—to move the candidate out of the zone of the known into the less known. It ought to display the candidate’s mode of thought, his or her tone and style especially frustrated—for politics is, among other things, a Mick Jagger world when you don’t always get what you want, and the public has the right to see how the candidate acts in that circumstance. The cause of public knowledge is not served if the questions crash into a familiar wall; if they elicit no more than the usual string of talking points; if they repeat what others have asked already in visible venues; if the answers only replicate the boilerplate in the stump speech or on the website.

Most of all, the public is not served when mistakes, distortions, and lies go uncorrected.

Politicians are more or less artful dodgers. To be useful, interviewers have to slip beneath their defenses. If they hear an evasion, they need to ask the question again. Fair’s fair: This gives the interviewee a chance to wriggle off the hook. And if not, it makes plain that they fail, or refuse, to talk straight. The cheap substitute for such explorations is the Gotcha moment. The more difficult way is to ask the question a different way, to root around, to explore motives and causes.

George Stephanopoulos’s best moment with John McCain this week came when the host asked McCain about what was, to me and several others (for example, James Fallows), the most peculiar and conspicuous physical fact of the debate.

Here’s how the moment went on ABC:

STEPHANOPOULOS: You know, during the debate, it seemed that you were reluctant to look at Senator Obama.

MCCAIN: I wasn’t.

STEPHANOPOULOS: No?

MCCAIN: Of course not.

STEPHANOPOULOS: Well, we went back through the tape, and some people were saying that that was showing disdain for him. Is that fair?

MCCAIN: I was looking at the moderator a great deal of time. I was writing a lot of the time. I in no way know how that in any way would be disdainful.

Friday night, it was not hard to see many moments when McCain was refusing to look at Obama but was not looking at the moderator or writing. Many. He was staring at the audience or at the camera. When Obama approached at debate’s end, McCain glanced at him for a fraction of a second, then looked away. Did Stephanopoulos miss these moments? If he saw, why didn’t he challenge McCain? Instead, the interview veered away from verifiable fact and went to this odd locution about his intention:

STEPHANOPOULOS: Certainly not what you intended?

A strangely helpful way to put the question, starting with “Certainly”—putting exculpatory words in McCain’s mouth. Why didn’t Stephanopoulos challenge McCain about where he was looking when he wasn’t looking at Obama?

McCain contradicted himself a moment later: “I don’t look at my opponents because I’m focusing on the people and the American people that I’m talking to.” So he was retracting his previous claim that he had been busy with the moderator and note-taking. Or, to be more generous, he was supplementing it. Perhaps he really didn’t know where he was looking. In any event, we lost an opportunity to hear McCain explain himself.

Here are some other roads left untaken during this interview:

• As during the debate itself, McCain made much of his antagonism to earmarks. Since the president’s only choice with earmarked bills is to sign them or veto them, was he pledging to veto every earmarked bill? More important, Obama made the case Friday night that earmarks amount to $18 billion a year, less than two months of the Iraq war, and a puny amount compared to the total budget, let alone the $700 billion figure tossed around as the cost of this week’s bailout. What does McCain make of that?

Todd Gitlin , who teaches journalism at Columbia, is the author of a new book, Occupy Nation: The Roots, the Spirit, and the Promise of Occupy Wall Street.