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Hillary Clinton is determined to get under Donald J. Trump’s skin at Monday’s debate, and is testing attack lines to try to rattle him.

Mr. Trump is largely shunning traditional debate preparations, but has been watching video of Mrs. Clinton’s best and worst debate moments, looking for her vulnerabilities.

The two candidates are taking vastly different approaches to what is expected to be one of the most widely watched presidential debates since Carter vs. Reagan in 1980. And their divergent strategies reveal how the candidates and their campaigns see the race, their strengths and their opponents’ weaknesses.

Mrs. Clinton has a thick dossier on Mr. Trump after months of research and meetings with her debate team, including analysis and assumptions about his psychological makeup that Clinton advisers described as critical to understanding how to knock Mr. Trump off balance. Mrs. Clinton has concluded that catching Mr. Trump in a lie during the debate is not enough to beat him: She needs the huge television audience to see him as temperamentally unfit for the presidency, and that she has the power to unhinge him.

Mr. Trump, in turn, is approaching the debate like a Big Man on Campus who thinks his last-minute term paper will be dazzling simply because he wrote it.

He has paid only cursory attention to briefing materials. He has refused to use lecterns in mock debate sessions despite the urging of his advisers. He prefers spitballing ideas with his team rather than honing them into crisp, two-minute answers.

With Mrs. Clinton largely devoting the next four days to mock debate sessions and drills in New York, and Mr. Trump hunkering down only on Sunday, here is a scouting report on the two prospects and their training regimens for Monday’s face-off, according to advisers, allies and friends of both candidates.

Preparations

Clinton:

She is mentally readying herself for multiple Trumps: the disciplined opponent who sticks to big themes, the no-holds-barred adversary who goes on the offensive, and the snide antagonist who calls her a “loser” to her face. Her advisers are hurling a host of Trumpian assaults and counterattacks at her to test her responses and adjust them as needed.

Mrs. Clinton is eager to play offense and try to get under his skin, by doing things like calling him “Donald” and questioning his net worth.

Yet she is also testing out whether and how to interrupt Mr. Trump, as she does not want to be seen as pushy and play into gender stereotypes. In practice sessions, she has come across best when she waits to pounce confidently on Mr. Trump for lying or misstating facts, rather than trying to talk over him.

Trump:

He does not like practicing an answer over and over until it is letter-perfect and appropriately brief. But this weekend’s work will be geared to running through questions while Mr. Trump is on his feet and aware of a countdown timer when he is speaking.

His advisers will try to throw him off balance, and measure his response to possible Clinton jabs like “You’re lying, Donald.”

He believes debates are not won or lost on policy minutiae since most viewers will not remember them in an hour. His advisers see it as a waste of time to try to fill his head with facts and figures. Instead, they want him to practice staying focused on big-picture themes (jobs, terrorism, protecting the homeland and closing borders, “Make America Great Again”) rather than picking fights on side issues or taking the bait from Mrs. Clinton.