“I knew I was communicating with someone who might not tell the truth about it if it ever became an issue,” Mr. Comey said. From that point on, Mr. Comey decided to document his one-on-one interactions with the new president in case there was ever any question about what had transpired during such sessions.

“Normally I wasn’t even much of a note taker,” he said. “But here I thought given those factors that I would need a detailed account in the event I need to draw on it in the future.” Which, of course, is exactly what happened.

Comey didn’t trust the Justice Department to conduct a Trump investigation itself.

I asked Mr. Comey why, after his firing in May 2017, he had deliberately shared the contents of one of his memos with a reporter, in the hopes of producing news coverage that would alarm the public about Mr. Trump’s conduct and therefore prompt the appointment of a special counsel.

Why did he decide to work outside the justice system, rather than within it? His reply was blunt: He did not trust the leaders of the Justice Department, including the attorney general, Jeff Sessions, and the deputy attorney general, Rod J. Rosenstein, to effectively and fairly carry out such an investigation themselves.

In his mind, Mr. Sessions would soon be recused from the Russia investigation, rendering him ineffectual, and Mr. Rosenstein had shown himself to be untrustworthy because of his role in firing Mr. Comey.

It was Mr. Rosenstein who had written a memo to the president laying out the reasons Mr. Comey should be fired. “I did not have confidence that it would be done in the right way,” Mr. Comey said.

Comey kind of wishes he were still the F.B.I. director.

“My plan was to stay,” Mr. Comey told me, when I asked if he wished he were still the director of the F.B.I. He might have eventually been fired over a different clash with Mr. Trump, he said. But as attacks on the F.B.I. have mounted, including from the president, Mr. Comey said, his desire to protect the agency has grown.