Chris Wallace knew what Donald Trump was going to say.

He knew that when he asked Trump about reining in government spending during a debate in March, the Republican presidential front-runner would boast about closing the federal deficit through substantial cuts to the Education Department and the Environmental Protection Agency. Wallace knew Trump would say this because he had said it before -- and the real estate magnate repeats himself often.

Wallace knew one other thing, too: Trump's calculations were way off. Even the total closure of both agencies wouldn't come close to eliminating the deficit.

So the Fox News Channel anchor ordered up a full-screen graphic to drive home the folly of Trump's bad and incomplete math. He did the same thing on three other subjects. As debate night approached, he was just about certain that he would catch the billionaire in his own nonsensical words.

Still, a twinge of doubt set in. Trump also prides himself on being unpredictable.

"One of my producers asked, 'What if he doesn't say one of those four things?' " Wallace recalled in an interview with The Fix. "I said, 'Well, then we're screwed.' "

They weren't screwed. Trump made the exact argument Wallace expected. And when he did, the moderator was ready.

"Mr. Trump, your numbers don't add up," Wallace said flatly.

Then came the pièce de résistance: "Please put up full-screen number four."

Moments later, Trump staggered into another set-up when he asserted that "you are talking about hundreds of billions of dollars" in additional savings from negotiating lower drug prices from pharmaceutical companies that sell to the government.

"No, you're not, sir," Wallace retorted. And again: "Let's put up full-screen number two."

On the Fox News telecast, the crowd in Detroit could be heard clapping and cheering.

"I think it's literally the only time a graphic has gotten an ovation at a debate," Wallace said, remembering the reaction. "It's kind of like what [Washington Post fact-checker] Glenn Kessler does — fact-checking a candidate — except I was doing it in real time in front of millions of people."

I loved the full-screens, too. Of course, they were soon derided by Trump supporters as "gotcha" questions — inquiries that can seem designed to make the respondent look bad. You might recall that Republican presidential candidates and party leaders flipped out last fall when they thought a CNBC debate featured too many "gotcha" questions. The national GOP even suspended NBC News as a sponsor of a future debate because of the episode.

So when I chatted last week with the host of "Fox News Sunday," which will mark its 20th anniversary on Thursday, I wanted to know how he justifies his approach to Trump — which wasn't out of the norm for one of the toughest interviewers in Washington.

His answer was very candid:

I think that the Sunday shows occupy a special place in the landscape of television news. It's the one place where you still get long, well-researched, probing interviews that aren't afraid to get into the weeds of policy, and look for and find contradictions between what somebody says now and what they said or did before. And, you know, part of that is good cross-examination technique, which is to ask somebody a perfectly open question but have a sense of what they're going to say and be prepared on the other side, if they say it.

He didn't have to talk about Medicare prescription drug prices; he just chose to do that.

Do I take a certain pleasure when I open the gate and he decides to walk down the path and I've got the bear trap at the end of the path? Yeah. Sure. But it's not just with Trump. We do it routinely, almost every week.

"Gotcha" questions can be unfair. Presidential candidates are not contestants on "Jeopardy" and shouldn't be set up for embarrassment by trivial quizzing.

But Wallace makes a strong case for the way "gotcha" questions can be used appropriately — like interview versions of sting operations — to hold candidates accountable for statements that are false, misleading or inconsistent with their previous positions. If a reporter can anticipate the fudgery that will come out of a politician's mouth, it doesn't indicate journalistic foul play. It indicates that the politician has been fibbing long enough to become predictable and is overdue for a fact check -- and that the journalist did his or her homework.

The path to the White House needs more bear traps, not fewer.