He said he was unsure if he would discuss his older brother Fred, who died as an alcoholic in 1981 at 43 — and whom he has described as an example of how destructive choices can damage lives that seem golden.

“Without my brother Fred I might not be here,” Mr. Trump said. “He was really smart, great-looking. I don’t drink or smoke because of what happened to him. I focused on building my business and making good choices. I may talk about that, but I don’t know if I should.”

Acceptance speeches seldom seem complete without anecdotes about personal trials and triumphs: Mitt Romney, trying to persuade voters to see him as more than a rich businessman, devoted about a fourth of his 2012 address to his parents’ unconditional love, his Mormon faith and reminiscences about watching the moon landing. In 2008, Barack Obama described how his grandfather benefited from the G.I. Bill and how his mother and grandmother taught him the value of hard work. And Bill Clinton’s 1992 speech vividly recalled the life lessons he learned from his mother about fighting and working hard, from his grandfather about racial equality — and from his wife, Hillary, who, Mr. Clinton said, taught him that every child could learn.

Mr. Clinton finished his speech with a now-famous line tying his Arkansas hometown to the American dream. “I end tonight where it all began for me,” he said. “I still believe in a place called Hope.”

James Carville, a senior strategist for Mr. Clinton’s 1992 campaign, said that if Mr. Trump hoped to change the minds of those who see him as divisive or bigoted, he would need to open himself up to voters in meaningfully personal ways in his speech.

“If he’s really different than the way he seems in television interviews or at his rallies, Thursday’s speech will be his single greatest opportunity to show voters who he really is,” Mr. Carville said.