Over the years, I have coached people on presentations to boards and councils. They often have prepared for complex questions and that is fine, in so far as it may help settle their nerves.
My emphasis, however, is on preparing them for the basic questions.
You can fumble or hesitate on a complicated subject, and no one will blink. Do so on a basic question, however, and your credibility begins to fly toward the window.
I am surprised at the lack of preparation that so many witnesses bring to Senate or House hearings. These people have been advised by DC wizards who should know better than to let them to punt on key topics.
The best questioners I have seen at the federal level are former Representative Barney Frank of Massachusetts*, Senator Ted Cruz of Texas, and Senator John Kennedy of Louisiana.
Videos of their techniques should be shown to potential witnesses.
*An old friend of mine who served in the Reagan administration once told me about his being shredded by Representative Frank during a hearing. Years had passed and he was able to see the humor in the moment.