With just days until the first Republican presidential debate, candidates are deeply immersed in preparation: boning up on policy, predicting opponents’ lines of attack, running through mock debates in underground bunkers. Right?

Well, not exactly.

“I’ve got a really fat book, and I’m sleeping on it at night, and I’m hoping all of that information will get inside my brain,” Sen. Rand Paul joked as he walked through the Capitol last week.

The first debate, to be hosted by Fox News on Thursday in Cleveland, will present an important opportunity for Republican candidates to introduce themselves to a wider swath of potential voters than any forum to date. But it won’t pose a daunting intellectual or rhetorical challenge.

Instead, the test will be of “who can craft the best one-liners and deliver them at the debate,” said Rick Santorum, who participated in the full calendar of Republican debates during the 2012 presidential election, and, despite being the runner-up for the nomination that cycle, won’t make the cut for Thursday’s prime-time stage, which is reserved for the top 10 candidates in an average of national polls. Instead, the former senator will spar among the six lowest-ranking candidates a few hours before the main event.

“All the candidates know that giving any kind of substantive answer is a mistake,” Santorum said. “And all you have to do is have zingers.”

As a result, the process of preparing for a primary debate, particularly at this early stage in the election cycle, bears little resemblance to the intensive regimen candidates undergo during the general election.

Whereas candidates often clear their campaign schedules in the lead-up to general election debates, most Republicans have in recent weeks kept up a full slate of speeches, primary state swings, and television appearances. With the prime-time roster to be released on Tuesday, candidates on the bubble, including New Jersey Gov. Chris Christie, have been aggressively hitting the media circuit in hopes of upping their poll numbers. They will even participate in a debate-like forum to be hosted by C-SPAN in New Hampshire on Monday.

Sen. John McCain, the Republican Party’s nominee in 2008, said campaigns are usually able to predict the questions the debate’s moderators will pose, “so you rehearse your answers.”

“In the primary, we just went over the questions that we knew there would be,” McCain said. “But there’s always one that you’re not ready for, and that is really what separates the ones who are capable and those who are rehearsed.”

At its most elemental, a candidate’s primary debate performance hinges on standing out among candidates who broadly share a worldview and whose résumés are similar. This week, with at least 10 candidates slated to appear on stage during the primetime Fox News debate, some will have only a few minutes to make an impression.

The largest obstacle to that end might be the candidate standing at center stage: Donald Trump, who is at once unpredictable, entertaining, and has vast experience crafting made-for-TV moments, notably as host of “The Apprentice” on NBC.

“I look forward to the debate on Thursday night & it is certainly my intention to be very nice & highly respectful of the other candidates,” Trump tweeted last week.

As the Republican candidate currently leading in the polls, Trump will be in the spotlight Thursday — and he has shown in previous public appearances that he is willing to lob searing, personal attacks at other candidates.

Roger Stone, a Republican operative who helped Ronald Reagan, Bob Dole and Jack Kemp prepare for their own debate appearances, has been among the team advising Trump on messaging for this debate. But it’s unclear to what extent Trump will heed his advisers’ suggestions. Trump often veers so far off script as to extemporaneously write one of his own, as in his stem-winding speech to launch his presidential campaign earlier this year.

And if Trump’s style is difficult for his own advisers to predict, the task is exponentially more puzzling for Trump’s Republican challengers.

“Imagine a NASCAR driver mentally preparing for a race, knowing one of the drivers will be drunk,” tweeted John Weaver, a senior strategist for Ohio Gov. John Kasich. “That's what prepping for this debate is like.”

Other campaigns and candidates were not eager to discuss how they will handle Trump on the debate stage. But the best tack might be to ignore him: If a candidate attacks Trump directly, Trump will be entitled to a 30-second response under the rules.

That will require discipline, one of the qualities stressed by experienced political debaters.

Ohio Sen. Rob Portman has not run for president himself, but he has been cast in the role of helping Republicans prepare for debates. Most recently, Portman has been tapped for general election debates, taking on the role of President Obama for both Mitt Romney and John McCain.

But Portman was first cast as a sparring partner during the 1996 Republican primary, taking on the role of Lamar Alexander in Bob Dole’s mock debate sessions. Portman took the assignment seriously, even wearing a flannel shirt in the style of Alexander’s trademark one.

In a primary debate, Portman told RealClearPolitics, “The challenge is to get your message across, so you have to be even more disciplined about deciding in advance what your message is and taking every opportunity to get that message across.”

“Otherwise, no one’s going to remember you,” Portman said. “If there are 10 people in a debate ... your message has to be really clear and you have to be disciplined. So, if someone asks you a question about an unrelated matter, you have to figure out how to get it back to your issue.”

For the lesser-known candidates in particular, there is the added pressure to create a memorable moment, even a manufactured one.

“That’s what debates are about,” said Hogan Gidley, a spokesman for Mike Huckabee. “They’re about creating moments.”

The best recent example of this dynamic might be the Republican primary debate in January 2012, when moderator John King opened the debate with a question for Newt Gingrich about whether he had sought an open marriage with his ex-wife, as she had publicly claimed.

"To take an ex-wife, and make it, two days before the primary, a significant question in a presidential campaign is as close to despicable as anything I can imagine," Gingrich responded memorably. The performance boosted his polling and gave his campaign a second wind.

But memorable moments can also work to a candidate’s disadvantage, if a candidate commits a uniquely egregious gaffe. During a Republican primary debate in Michigan in November 2011, Rick Perry did just that: uttering “oops” when he could not name a third federal agency he hoped to eliminate.

On Thursday, Perry, who is on the bubble for the top-10 debate, will still be working to recover from that mistake.

“I feel comfortable that we’re going to go up there and perform in a very capable and solid way, and at that particular point in time, 2012 will be history,” Perry told RealClearPolitics recently. “I kind of look forward to it, actually.”

For Perry and other Republican candidates, the simplest objective Thursday might be to make a positive first — or second — impression, and do no harm.

“No one is going to close a sale in this debate, or should try to,” said Stuart Stevens, who was Mitt Romney’s top strategist during the 2012 election. “It's the start of long interview process. This isn't the Hunger Games — it's not going to be one person left standing at the end."

Rebecca Berg is a national political reporter for RealClearPolitics. She can be reached at rberg@realclearpolitics.com.