Before any of them takes the debate stage on Tuesday and Wednesday, the 2020 Democratic candidates will already have put in dozens of hours each preparing. But here’s what they need to understand: Nothing they do to prepare for the primary debates will feel remotely similar to when the nominee faces Donald Trump in a debate.

My perspective on this is unique: I was assigned the role of playing Trump during Hillary Clinton’s general election debate prep in 2016. His stand-in. Her sparring partner. The ball machine. Over 17 years of working for her, I’ve argued with and annoyed Hillary plenty. But this time, I was supposed to. And without question, she did the debates right. They were her most successful three days between the high of accepting the nomination and the low of Election Day.

To prepare myself for her grueling debate prep, I watched the 15 Republican primary debates and forums in which Trump participated three times each: once the whole way through; a second time focusing entirely on the exchanges he was part of; a third time with the sound off to watch his mannerisms and body language. I might know his debating style—if you want to call it that—better than anyone on the planet (aside from Hillary Clinton, of course). […]