“There were a lot of special guys and there were some challenges, but there was also a lot of personal growth,” Belichick said of his years with the Giants. “Me, too. I was 26 or 27. I had a lot of growing to do as a person and as a coach.”

With the Giants, Belichick began developing the coaching style that is fully formed now, down to the cut-off sweats and sneakers that the Hall of Fame linebacker Harry Carson remembers Belichick wearing even then.

Mara recalls Belichick at practice being like an intense college professor, always teaching and rarely screaming. Belichick was a stickler for detail, and he preached the importance of each player knowing his role. Carson’s job was to stop the run. Taylor’s was to rush the quarterback. There was no ambiguity, Carson said. Not much has changed. Only a few weeks ago, Belichick was caught on a field microphone exhorting a group of Patriots players to “just do your job.”

“We always tried to whittle it down to the smallest possible things,” said Virginia Coach Al Groh, who coached the linebackers for two years when Belichick was the Giants’ defensive coordinator and remains close to Belichick. “We kept narrowing the focus on what we were going to give them. Players were focused not just from a mental and emotional standpoint, but from a specific task standpoint.”

The Browns hired Belichick right after the Giants won the Super Bowl in January 1991. Two years earlier, Accorsi, Cleveland’s general manager at the time, had interviewed Belichick at the Senior Bowl. The Browns had already decided on Bud Carson to be their coach then, so Accorsi conducted the interview in a coffee shop. Another man was waiting for Belichick, so they could go watch the practices together. After two hours, the other man left. After six hours, the interview finally ended.

“The interview was so captivating,” Accorsi said. “He was so prepared and had such strong convictions, everything from strategy to the way he saw the game, to player personnel. He had all kinds of theories and innovations. And he doesn’t just pick up players because they have talent; he has a job for them.”

Image Bill Belichick, carried off the field after the Giants won the 1986 N.F.C. championship game, still has a soft spot for the team. Credit... The New York Times

When Belichick wanted to keep linebacker Junior Seau from retiring last year, he called Seau in San Diego. “I have a job for you,” Belichick told him.