George Mitchell was brokering a peace deal in Northern Ireland when Bill Clinton called upon the former Senate majority leader for a favor. It was the summer of 1996, and the president, who was mere months away from re-election, needed Mitchell’s help. His first presidential debate against Bob Dole was fast approaching, and President Clinton, eager to prepare, wanted the seasoned politician to play his Republican rival in a series of mock debates. Mitchell obliged. He pored over binders full of Dole’s public statements over the years, then flew to Chautauqua, New York, where Mitchell and Clinton proceeded to go head-to-head, every night for three straight nights. “Mitchell came in and just demolished Clinton in the mock debates at first,” Clinton’s chief speechwriter at the time, Michael Waldman, recalled. “None of the reporters believed us when we told them that. They thought that it was just spin to lower expectations, but it was really true. But by the end, it forced Bill Clinton to focus in on what he wanted to say.”

Now, 20 years later, Clinton’s wife, Hillary Clinton, is preparing for a historic presidential debate of her own—and she hasn’t taken any chances. Ahead of her hotly anticipated, nationally broadcast showdown with Donald Trump on Monday, the former secretary of state has worked tirelessly with a cast of big-name political operatives that includes Ronald Klain, Joel Benenson and Karen Dunn. While her Republican opponent has reportedly declined to hold mock debates or formal strategy sessions, instead reportedly meeting with Roger Ailes and Rudy Giuliani for Sunday joke-writing sessions over cheeseburgers and hot dogs at Trump’s New Jersey golf course, Clinton and her team of Beltway professionals have diligently dissected Trump’s primary-debate performances to exploit his weaknesses. As The New York Times reports, Clinton and company have even enlisted the help of Tony Schwartz, the ghostwriter of Trump’s book The Art of the Deal, to better understand the real-estate mogul’s psyche. But practice makes perfect, according to presidential-debate experts who have coached former candidates and presidents, and mock debates are ultimately what separates successful debate performances from those that make history for less positive reasons.

As televised debates have become the critical, final test of presidential candidates, so has debate practice become standard practice for presidential campaigns. Few remember the specifics of Ronald Reagan and Walter Mondale’s second 1984 debate; but everybody knows Reagan’s famous quip, “I will not make age an issue in this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience.” With the value of a well-timed zinger firmly in mind—particularly in the social-media era—preparation for the staged face-offs has become an arduous, albeit formulaic, process, not only for the candidate, but for the stand-in, who must, as Mitchell did, invest weeks of time so they can accurately represent the opposition in a debate scenario.

“I made it very clear to President Clinton that I would make no effort to emulate Senator Dole’s mannerisms but rather would do my very best to capture his words, and I think we did that,” said Mitchell, who debated Dole on countless occasions when they served together in the Senate. “Every single word spoken by Senator Dole in the debates, President Clinton had previously heard in the debate preparation from me,” he told me. “That is, we, in the process of the preparation, were able to anticipate what he would say based upon what he had said up until that time. And I think that was probably true in reverse. There is no unique magic to this.”