And in President Bill Clinton's 1999 impeachment trial, three witnesses were called to testify: Monica Lewinsky, Clinton confidant Vernon Jordan and White House aide Sidney Blumenthal. These witnesses all sat for videotaped depositions conducted by the House managers and then relevant portions as determined by the Senate were played for the senators.
All this occurred after an exhaustive investigation by special counsel Kenneth Starr that began in 1994 and lasted until he delivered his voluminous report in 1998. During that four-year period, Starr was able to question under oath every witness he wanted from Clinton's close aides to even President Bill Clinton himself.
In contrast, in Trump's case, the President directed certain key witnesses "not to comply," with subpoenas. Trump feared what they would say under oath.