The second of John McCain and Barack Obama’s debate matchups prove just how much assumptions are a dangerous business.

Falling behind on the polls, McCain was the one who was supposed to be on the attack. Instead, it was Obama who offered the first bite, and one of the stronger blows of the evening. “This is the guy who sang bomb, bomb Iran,” Obama said, referring to a now famous YouTube video of his rival.

The town-hall format was supposed to favor McCain. But it was Obama who seemed slightly more at ease.

And this was supposed to be a possible turning point in the election, a debate that could solidify it toward Obama or inject a new sense of momentum for McCain. Instead, the evening plodded along, most zingers fell flat, and even the audience looked bored. One man fell asleep.

Who knew how unexciting average Joes could be in this format?

That was the makeup of the audience of 80 undecided voters who gathered in Nashville for this forum, with Tom Brokaw guiding them through their host of worries and concerns. But missing was any sense of real energy, whether from the candidates or their inquisitors, save for Brokaw in his attempts to keep things on time.

Perhaps it should have been no surprise that no one brought up Bill Ayers or Charles Keating, the nefarious figures from Obama’s and McCain’s past that each candidate has tried in recent days to make an issue.

Instead, this debate was about real things, and usually when that happens it is hard to keep the candidates from veering at all off of talking points.

With the Dow continuing to plunge, the credit markets still frozen and Depression-analogies becoming ever-more-so common, the debate may have said more about how the two candidates are struggling to respond to the changing dynamics now well beyond their control.

Both candidates spent much time at the start of the evening trying to explain the bailout, and just how it would help the regular people in the Nashville audience.

Concerns about paying the mortgage, paying for college or paying for retirement quickly got enveloped in the candidates’ desires to bring the conversation back to portions of their own stump speeches.

Little is any mention was made of the plunge in the stock markets.

Will things get worse? One questioner asked.

Obama: “No, I am confident about the American economy,” he said, before going into health care and energy and removing special interests from Washington.

McCain: “I think it depends on what we do,” he said, before again citing his proposal to buy up bad loans and to remove special interests from Washington.

In fact, moderator Brokaw’s battle to keep them within their respective time limits was ultimately a losing one, a function of both contenders’ desires to not lose an inch.

Obama was perceptive and even a bit calculating, and seemed enough at ease as to not appear overly defensive even when McCain did hit back. Several times, he said he “agree with Sen. McCain” — the subject of some criticism after the first debate — but he combined it this time with some swipes as well. What was more apparent was the age differential, and Obama offered some subtle digs at the generation gap. When Brokaw admonished both candidates on taking too much time, Obama said, “I’m just trying to keep up with John.” It was like an amusing comment a grandson would make to a shuffling grandfather.

McCain did appear nervous at points. He paced back and forth during answers to questions, peered at notes in giving answers and even offered one hint of his awkward grin after ending an answer. The latter has been a just a bad habit. And his attempt at humor, like a comment about needing hair transplants, fell a bit flat. His reference to Obama as “that one” ricocheted through liberal blogs as further evidence of McCain’s contempt.

But McCain was emphatic on what he called a more measured approach to foreign policy — apparently to buck hardening perceptions that he is erratic.

Twice McCain cited his opposition to “my hero” Ronald’s Reagan’s decision to send troops to Lebanon in the early 1980s. “Unfortunately, almost 300 brave young Marines were killed,” he said.

He chided Obama for “telegraphing” his intention to pursue Osama bin Laden in Pakistan even if it meant crossing the country’s borders without permission. McCain paraphrased his hero Teddy Roosevelt and said, “Use force, but talk softly and carry a big stick.”

You know things are different when it’s the Republican accusing the Democrat of being the warmonger.

Obama had vowed that his administration “would kill Bin Laden” and “crush Al Qaeda.” and replied, “Sen. McCain, this is the guy who sang, ‘Bomb, bomb,bomb Iran, who called for the annihilation of North Korea. That I don’t think is an example of ‘speaking softly.’”

It was a rare unexpected moment, in a debate that never really did live up to expectations.