George H. W. Bush checking his watch in 1992. Al Gore invading George W. Bush's personal space in 2000. John McCain pointing at Barack Obama and derisively calling him "that one" in 2008.

Thought those were tripwire moments in the annals of presidential town hall debates? Just imagine the reaction if Republican Donald Trump on Sunday is forced to address his lewd 2005 hot-mic banter about groping women "by the p—sy" to a roomful of undecided voters.

Watch tonight's presidential debate with CBC News. Scroll down for details.

Trump and Democratic nominee Hillary Clinton meet again at 9 p.m. ET on Sunday, without their lecterns, for the second presidential debate at Washington University in St. Louis, MO.

Only this time, there's something else hanging over the proceedings: The impossible-to-ignore recordings about Trump's sexual advances, in which he bragged to Access Hollywood host Billy Bush that he "did try and f—k" an unnamed married woman, later revealed to be Entertainment Tonight host Nancy O'Dell.

WARNING: This video contains graphic language

Damning as the 2005 tape has become after its release on Friday, how the whole incident plays before a crowd of uncommitted voters could get downright uncomfortable. The town hall format can be thorny enough as it is.

"No pun intended in terms of the Republicans, but this recording is the elephant in the room," says Mitchell McKinney, one of the original consultants who worked with the Commission on Presidential Debates when the first town hall format was rolled out 14 years ago.

McKinney expects the moderators to ask Trump to address the controversy early in the debate, rather than to place that burden on voters who will get to question the candidates. (Trump offered a brief video apology late Friday, but also used the occasion to frame it as an attack on Clinton and her husband Bill Clinton's infidelities.)

Here is my statement. <a href="https://t.co/WAZiGoQqMQ">pic.twitter.com/WAZiGoQqMQ</a> —@realDonaldTrump

Trump's response, "whether it's a contrite, sincere apology," or whether he emerges shame-free and turns it around as an attack on the Clintons, could still lead to red faces in the crowd, McKinney says.

"How that all gets done could be awkward in front of these undecided citizens."

While the first debate offers a window into how the candidates respond to attacks against each other, town halls highlight how they relate to, empathize with, and respond to citizens' often straightforward, pocketbook concerns.

'Attack-oriented' approach unwise

No doubt following the hot mic incident, questions are also being rewritten or slotted in to raise the subject of Trump's response to the leaked recordings, which were first obtained by The Washington Post.

Sunday's 90-minute, commercial-free showdown will be co-moderated by ABC's Martha Raddatz and CNN's Anderson Cooper. Moderators will pose half the questions. Pre-screened audience members will ask the candidates about the most pressing issues affecting their lives.

Controversies aside, town halls can be "very tricky" forums, says McKinney, the director of the Political Communications Institute at the University of Missouri.

"Unlike the podium debates, the candidates in the town hall debate do much more performing. They're on a stage, they're walking about, they're sometimes relating to or interacting with undecided voters."

Retail politics will come into play, perhaps not Trump's forte. The New York businessman tends to feed off the energy from boisterous rallies teeming with thousands of supporters.

U.S. Republican presidential candidate Donald Trump will need to relate to an intimate crowd of fence-sitters. (Patrick Semansky/Associated Press)

In St. Louis, he'll encounter an intimate crowd of fence-sitters and will need to demonstrate a more level-headed disposition.

The uncommitted voters at the town meeting will have been chosen by the Gallup Organization. The candidates will have two minutes to answer the questions, followed by one minute of broader discussion.

Has he practised ? Will he come in more with a specific plan than in the first debate? - Alan Schroeder, author of Presidential Debates

"If he's too attack-oriented, if he's easily goaded, if he flies off the handle, that's a very difficult style to pull off in a town hall debate," McKinney says.

After his widely panned performance against Clinton in the first debate, Trump suggested he might "hit her harder" in a future match-up, alluding to the possibility of referencing Bill Clinton's infidelities.

Probably not a smart way to go, McKinney warns.

"It can be seen as unseemly," doing so while standing in front of undecided citizens, and would reinforce a narrative "that he lacks the presidential temperament or persona, or that he's unfit or too hotheaded, too rash, too much of a bully."

That's why Trump is unlikely, unless baited, to return to his criticism of former Miss Universe Alicia Machado, who has emerged as a very public critic of the Republican candidate. (Trump has urged followers to check out her "sex tape" — a night-vision clip from a Big Brother-type reality TV series. It shows movement under bedsheets, but no nudity.)

Did Crooked Hillary help disgusting (check out sex tape and past) Alicia M become a U.S. citizen so she could use her in the debate? —@realDonaldTrump

A warning about canned material

Clinton has her own challenges, one being her need to overcome perceptions that she's too wooden or cold.

"The likability problem," as McKinney puts its. "As she's trying to relate to these citizens, the challenge is she won't appear comfortable, [she'll be] stilted and won't break out of that to seem warm, friendly, relatable in interacting with these citizens."

The forum of a town hall is just not the appropriate venue for canned material - Mitchell McKinney, University of Missouri

Over-preparation, in Clinton's case, could be a liability this time, if it interferes with her ability to connect with the audience and overcome dismal trustworthiness ratings.

Preloaded zingers such as her "Trumped-up, trickle-down economics" line from the first debate could fall flat, says Aaron Kall, the director of debate at the University of Michigan.

"The forum of a town hall is just not the appropriate venue for canned material," he says, noting that undecided voters with the opportunity to personally question a candidate "will demand the entire attention span of the candidate."

Democratic presidential candidate Hillary Clinton will be challenged to come across as less wooden. It would be a mistake to pull out a series of canned zingers. (Matt Rourke/Associated Press)

That means they'll want thoughtful responses, not answers "filled with things that have been thought about in advance that sound forced and phoney."

The St. Louis crowd, as well as audiences at home, will see right through an overly scripted style, says Alan Schroeder, author of Presidential Debates: Risky Business on the Campaign Trail.

Town halls serve a purpose as humanizing stages as well. With real voters present instead of "an abstract audience on the other side of a television camera," Schroeder says. "This is a ready-made opportunity for Trump and Clinton to express a more human side of themselves."

Trump coached by a master of town halls

Clinton has not been viewed as a gifted retail politician, though her quarter-century in the public eye, as well as being a former first lady, New York senator and secretary of state could lend her the advantage of experience.

With Trump, the question may be how he has re-strategized.

"Has he practised? Will he come in more with a specific plan than in the first debate? Will he be able to tamp down some of the things that got him intro trouble, like taking the bait that Clinton dangled in front of him?" Schroeder asks.

LIVE DEBATE COVERAGE Watch the debate live on CBCNews.ca, CBC News Network and CBC Radio One starting at 9 pm. ET Sunday. CBC News reporter Matt Kwong will be taking your questions on our live blog. Join Rosemary Barton and the team from Power and Politics for ​highlights and analysis after the debate from 10:30 to 11 p.m. ET on CBC News Network and CBCNews.ca.

Trump, who reportedly showed little interest in prepping for the first debate, appears serious about rebounding. His last-minute scheduled town hall in New Hampshire on Thursday three days before Sunday's face-off gave him the opportunity to familiarize himself with the format. He has also reportedly had coaching from New Jersey Gov. Chris Christie, a master of town hall campaigning.

"It's an indication his advisers are trying to get him in the right frame of mind," Schroeder says.

Trump has the lower bar but, with a strong performance from his running mate Mike Pence in the vice-presidential debate, there is chance to ride the momentum.

Clinton has a burden, too.

"She's done very well on the first debate, so now people have high expectations for her performance to be of equal or better quality. That's her challenge as well, to deliver another win for the team."

Watch The Choice 2016 on The Passionate Eye on Sunday October 16 at 10 p.m. ET & PT on CBC News Network for new insights into Donald Trump and Hillary Clinton and why they both want one of the most difficult jobs imaginable.