LOS ANGELES — It’s the moment that Judith Hill has been replaying in her mind for the last two months: She was sitting on a plane with a man she loved, talking, having dinner, when suddenly he lost consciousness. She shouted his name: Prince. She shook him. But he didn’t come to.

Her swift reaction may have helped save Prince’s life that night, six days before he died of an accidental overdose of the opioid painkiller fentanyl.

“His eyes fixed,” just before he nodded off across a table from her, Ms. Hill, 32, recalled in an interview here, speaking for the first time about her presence on the April 15 flight from Atlanta, after Prince’s two shows there. Only one other passenger was on the private jet, Prince’s longtime friend and aide Kirk Johnson. They were bound for Paisley Park, Prince’s estate outside Minneapolis. Over vegetables and pasta, Prince and Ms. Hill discussed his performances that night, which turned out to be his last public concerts; other musicians like the funk singer Betty Davis; and photography, one of Prince’s hobbies.

According to flight-tracking reports, the chartered 1988 Dassault Falcon 900 took off at 12:51 a.m. from Hartsfield-Jackson Atlanta International Airport and was near Chicago, less than an hour from its destination, when Ms. Hill witnessed Prince fall unconscious. If she had glanced away in that instant, down at her phone or purse, she might have thought he had simply dozed off. “Thankfully, I happened to be looking into his face,” she said.