By Jon Keller

BOSTON (CBS) — In a campaign that has taught us to expect the unexpected, tonight’s second presidential debate in St. Louis may well provide some of the most unexpected moments yet.

It’s the one and only town hall-style debate, with forty undecided voters onstage along with Hillary Clinton and Donald Trump, sharing questioning duties with moderators Anderson Cooper and Martha Raddatz. And from the very first one back in 1992, the addition of citizen questioners has provided some of modern history’s most memorable debate moments.

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Bill Clinton said he was thrilled to have this format included after having done many town hall sessions during his run to the nomination, and that 1992 quickly showed why. A woman rose and asked: “How has the national debt personally affected each of your lives?”

Incumbent President George H.W. Bush answered first.

“It has a lot to do with interest rates,” he began, but the woman cut him off: “How has it affected you personally?”

Bush, a millionaire, seemed nonplussed. “Well I’m sure it has, I love my grandchildren and hope they’ll be able to afford an education,” he said.

When it was Clinton’s turn, he approached the woman and began his answer with a question.

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“You know people who’ve lost their jobs, lost their homes?”

Yes, she did. And Clinton was off and running with a far more empathic answer than either Bush or Ross Perot, the third candidate onstage, could muster.

So tonight, watch to see how Hillary Clinton and Trump handle the theatrics of the moment. Who does the best job of establishing a personal connection with the citizen questioners, and handling it if they push back on their answers?

And what if a citizen pops a question that targets a candidate’s political weakness, such as Clinton’s e-mail follies or Trump’s ill-advised litany of derogatory comments about women? As veteran New Hampshire political expert James Pindell of the Boston Globe puts it, “what happens if a larger woman gets up and says ‘Donald Trump, am I fat, and what do you think of me?’ That is an amazing moment.”

History suggests the professional moderators are more likely to raise so-called “gotcha” questions than the citizens. But it will be interesting to see how the candidates react to the citizens and vice-versa. Trump has already indicated he will not follow the advice of some to raise Bill Clinton’s checkered personal history as an issue, likely because he fears a negative reaction from the onstage audience.

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In any case, there’s one thing I cam promise you about tonight’s debate – I’ll have highlights, lowlights and analysis for you at 11pm on WBZ News.

Jon Keller