I've Watched Trump Testify Under Oath. It Isn't Pretty. In a 2007 deposition in my libel case (which he lost), he was underprepared and overconfident.
Excerpt: Whether he realizes it or not, Trump is in a perilous position. He presides over a chaotic White House stocked with competing interests and egos, he's mired in a complex investigation and he's advised and protected by a relatively scanty phalanx of private attorneys. If the president goes mano-a-mano with Mueller, the outcome of that encounter is likely to hinge on how careful, credible and capable he is under oath. Speaking from experience, I think the president's attorneys should grab their worry beads. Trump sued me for libel in 2006 for a biography I wrote, "TrumpNation," alleging that the book misrepresented his business record and understated his wealth. Trump lost the suit in 2011, but during the litigation my lawyers deposed him under oath for two days in 2007. We had the opportunity to ask Trump about his business and banking practices, his taxes, his personal finances and his professional relationships. Trump's attorney then was Marc Kasowitz, who also briefly represented the president when the Justice Department investigation first got rolling in Washington. My attorney was Mary Jo White, a former federal prosecutor steeped in many of the same legal traditions and courtroom experiences as Mueller. It didn't go well for the future president. Hammered by White and her deputies, Trump ultimately had to admit 30 times that he had lied over the years about all sorts of stuff: how much of a big Manhattan real estate project he owned; the price of one of his golf club memberships; the size of the Trump Organization; his wealth; his speaking fees; how many condos he had sold; his debts, and whether he borrowed money from his family to avoid going personally bankrupt. He also lied during the deposition about his business dealings with career criminals.