 In 2016, I prepared intensely for the debate, says Hillary Clinton, because I knew I had to find a way to cut through Mr. Trump's antics and help the American people understand what was really at stake. In 90-minute mock debates on an identical stage, I practiced keeping my cool in the face of hard questions and outright lies about my record and character. A long-time advisor played Mr. Trump and did everything he could do to provoke, rattle, and enrage me. It worked.