The veteran CBS reporter, who moderated three presidential debates, looks back at key moments from those clashes and gives advice to his 2016 counterparts. (Adriana Usero,Erin Patrick O'Connor/The Washington Post)

Bob Schieffer, a political contributor to CBS News, hosted “Face the Nation” from 1991 to 2015.

I moderated presidential debates in 2004, 2008 and 2012, and it’s an experience like no other.

People don’t believe me when I say I don’t get nervous on television, but I don’t. I’ve done it so long that it has become second nature to me — like a veteran ballplayer getting so zoned in that he forgets he’s playing in front of a crowd.

But as I was standing backstage before my first debate — George W. Bush and John Kerry in 2004 — I looked down at my hands and realized I was shaking like a leaf. I hadn’t had stage fright in 30 years. Yet, as I stood there thinking how the race for the most powerful office in the world could be influenced by what happened that night while millions upon millions of people were watching, it made me a nervous wreck.

Then it came to me. For sure, this was the World Series of politics, but the people who should be nervous were the players. I was just the umpire. With that in mind, I settled my own nerves.

It’s also why I believe the most important advice I can give to a moderator is this: Remember that nobody goes to a ballgame to watch the umpire. We’re not electing a national moderator; we’re electing a president. If you keep that in mind, the whole thing gets easier.

Over the years, I’ve been asked to define the moderator’s role. To me, the moderator’s main responsibility is to give the viewers a better and more complete understanding of who the candidates are, and it’s more than where they stand on issues.

A vote for president is different from any other vote we cast. We often vote for local government candidates strictly on issues, such as Candidate A wants to put a stoplight at a certain intersection. If we’re for that, we vote for him; if we’re against it, we vote for the other person.

Not so for presidential elections.

Issues are important. Party affiliation is important. But in presidential elections, I believe most Americans cast their votes for the person they feel most comfortable with in time of crisis.

So the moderators must do their best to help viewers understand not only where the candidates come down on the issues but also how they react under pressure, whether they seem to know their stuff and whether they appear to have the poise and, yes, the courage to handle the awesome responsibilities of the presidency.

In the 2004 debate, I noted that like me, both Bush and Kerry were married to smart women and had two daughters, so I asked, “What is the most important thing you’ve learned from these smart women?”

Bush said “to listen to them.”

Referring to his wealthy wife, Kerry brought down the house when he said, “The president and you and I are three examples of lucky people who married up. And some would say maybe me more so than others.”

I thought it showed a side of both men we hadn’t seen.

A good laugh also keeps people awake.

One of the toughest assignments for the moderators is fact-checking in real time when they don’t have the luxury of looking something up. All the moderators this year are veteran journalists, so I’m confident they will have done their homework and be familiar with the issues before the debates.

But it’s more complicated than that.

I believe the chief fact-checkers are the candidates. If one of them says something that is dead wrong or inconsistent with what he or she has said previously, the other candidate should have the first opportunity to call his or her opponent on it.

This gives the viewers a chance to judge how knowledgeable both of the candidates are.

If neither candidate catches the inaccuracy, then the moderator must step in, set the record straight and, if necessary, ask a question about it. With more and more misleading, distorted and downright wrong information finding its way into campaign dialogue this year, moderators should be prepared to say, “Candidates, for the record, there is no evidence to support that,” or words to that effect.

The moderators’ greatest challenge in this year’s debates may not be fact-checking but keeping the candidates from hogging the 15 minutes allotted to discuss each of the 10 topics.

The last two debates I moderated, I sat at a table with the candidates. Having them in close proximity made reining them in easier. Maybe I’m wrong, but I also thought it made them more reluctant to hurl personal insults.

Hillary Clinton and Donald Trump so despise each other, I doubt either would agree to sit at the same table.

If they do get out of line, the moderator must remind them — forcefully, if necessary — that voters expect them to abide by rules they both agreed to.

And one final tip to moderators: If the candidates start throwing things, just dive under the table. Considering what we’ve seen this year, we’ll all understand.