WASHINGTON - Ask Hillary Clinton or Donald Trump how they are preparing for the debate Monday that could upend the presidential race, and they might just change the subject.

Their aides, of course, are handing them briefing books detailing where the cameras will be placed and what the price of milk is. They’re studying each other’s hand gestures and talking points on endless streams of video. They’re acting out scenarios designed to replicate the conditions onstage at Hofstra University.

The campaigns are cagey about preparations as they aim to lower expectations, refusing even to say who they’re using as stand-ins to role-play the other candidate, if anyone. But the nominees are leaving little to chance leading up to what is often the most dramatic, and least predictable, moment in a presidential election.

“It’s like preseason and regular season in football,” said Chip Englander, a GOP consultant who advised Kentucky Sen. Rand Paul and then Florida Sen. Marco Rubio in the primary. “You can simulate, but there’s nothing like the real thing. Do I think it helps? Yes. Do I think it perfectly replicates it? No.”

Even the participants are loath to predict how this 90-minute spectacle projected to break viewership records will play out. “I do not know which Donald Trump will show up,” Clinton said during a fundraiser a few weeks ago.

The erratic bomb-thrower? The restrained statesman? The conspiracy theorist? Even Trump isn’t sure. “If she treats me with respect I’ll treat her with respect,” Trump told Fox News. “You’re going to have to feel it out when you’re out there.”

Every one of his personas was present at the primary debates. Clinton has been feverishly watching the footage, studying Trump’s mannerisms and reactions as rivals tried to bait, belittle and even befriend him.

A general election debate is a very different arena. The nominees will tangle on the same stage for the first time. The kind of missteps during primary debates from which contenders quickly recover can prove devastating, even disqualifying.

Both Clinton and Trump, candidates who are deeply disliked by the electorate, are confronting what may be a final opportunity to redefine themselves. Clinton needs to overcome questions of trustworthiness. Trump needs to overcome questions of instability.

“A real boring debate is a huge win for him,” said Barry Bennett, a former senior campaign adviser to Trump who ran Ben Carson’s campaign during last fall’s primary debates.

Veterans of the process say the best moments, the ones that could potentially alter voters’ perception of the candidates, require preparation. But it’s a delicate balance: Look like you’re trying too hard to stage a moment and it comes off as phony. Wisconsin Gov. Scott Walker bombed with his “We don’t need an ‘Apprentice’ in the White House” debate line before dropping out of the primaries.

“Sometimes you hear these things and you think, ‘Someone who thought he or she was too clever by half wrote that late at night, when they were too tired to think,’” said Kathleen Hall Jamieson, a longtime scholar of presidential debates at the University of Pennsylvania.

Both the schedules the candidates have kept in recent days and their past habits suggest Clinton is spending more time in debate cram sessions than Trump.

But aides have told reporters that Trump has spent recent weekends prepping at his country club in Bedminster, N.J. His close circle of advisers includes Roger Ailes, who recently resigned his post at Fox News amid sexual harassment allegations and previously had a long career advising GOP presidential candidates.

“The key to these things isn’t so much being able to answer the question, but being able to answer in 30 seconds, which is not human nature,” Bennett said. “Roger’s got a lifetime of doing that.”

Trump says he worries overdoing it on prep could cost him the spontaneity that has served him well. Clinton is expected to have multiple dress rehearsals. It is unclear whether Trump will have any.

Even so, mock debates have proved invaluable to other candidates. Rick Tyler, who portrayed Trump against Texas Sen. Ted Cruz during the primary, became a method actor for two weeks, trying to embody not only Trump’s rhetorical style but also his mannerisms and thinking. He recalled the room falling silent after his first performance, during which he was combative, dismissive and ultimately victorious. It sobered some in the Cruz camp to the kind of threat Trump posed.

Republicans who have faced Trump offer advice. “Donald has a weakness. He does not want to be seen as uninformed or unintelligent or diminished in any way,” Tyler said. “It’s very tricky. You have to kind of mock him but you can’t diminish yourself while doing it.”