But the audience of undecided voters who will ask half of the questions on Sunday will not be as friendly as the supporters who asked Mrs. Clinton questions at campaign stops. Her efforts to seem sincere and personable may also collide with Mr. Trump’s efforts to score points against her for her use of a private email server as secretary of state; her oversight during the attacks on the American consulate in Benghazi, Libya; and her ties to wealthy donors to her family foundation.

“It helps if you’ve been subjected to questions from normal people during the campaign, which of course Donald Trump has not, but it’s not the same,” said Mickey Kantor, a longtime adviser to former President Bill Clinton.

Mr. Kantor said that having a moderator, an opponent and a television audience of potentially 80 million viewers made it almost impossible to prepare.

“You can’t just take a question and run off to your talking points,” Mr. Kantor said.

Advisers say Mr. Trump must alternate between personable and piercing during the 90-minute debate. They want him to show interest, empathy, and even some humility, in his exchanges with the audience, while hammering Mrs. Clinton in a way that is not too off-putting.

“It’s just harder to attack when nice, undecided voters are all around you,” Mr. Giuliani said. “It’s easier to do when people are sitting in a dark hall and you can barely see them.’’

Mr. Trump logged some practice on Thursday in New Hampshire by participating in a town hall-style forum that even included a two-minute clock — the same time limit he will face on Sunday. Mr. Trump denied that the event was a dry run for the debate, saying such a suggestion made him look “like a child.’’

His performance was uneven, even facing an audience asking friendly questions. Some of his answers were pointed, like his criticism of the Iran nuclear deal. Others were perhaps reassuring, like his insistence that he would stop the flow of heroin into the state. At other times, Mr. Trump meandered, such as a long response about Hispanics that included an observation about those in Las Vegas. “Latinos, they like to be called in that area,” he said.