Fox played Marty McFly in Back to the Future (1985) and its sequels. On The Tonight Show, Fox explained why watching the movie with Diana turned into a "nightmare."
"She was sitting next to me. The lights go down and the movie starts and I realize I'm one fake yawn and arm stretch away from being on a date with the Princess of Wales," Fox joked.
"But then what happened is the movie started and all the sudden I had to go pee. For the rest of the movie, I'm sitting there dying," he recalled.
"I can't say anything to her and I can't walk away from her because I can't turn my back on her, so it was just agony," the star said. "It could have been the greatest night of my life. It was just a nightmare."
Fox said Diana nevertheless seemed "ebullient" and appeared to like the film.
"She seemed to laugh a couple times," he said.
Fox shares other stories from his life in his memoir, No Time Like the Future: An Optimist Considers Mortality, released Tuesday. The actor discusses how breaking his arm in 2018 led to his "darkest moment" and caused him to question his optimism.
In 2018, Fox underwent a surgery unrelated to his Parkinson's disease battle to remove a noncancerous tumor from his spine. Fox was recovering from his surgery when he ended up falling while alone in his apartment.
During Monday's episode of Good Morning America, Fox said he'd reached a sort of "peace" with Parkinson's disease before the setback. He said he blames himself for the "careless" accident that led to him breaking his arm.
"It was so useless. lt was so pointless and so stupid, so avoidable," the actor said. "I didn't do anything to cause Parkinson's. I didn't do anything to cause the tumor in my spine. But I did this."
Fox said he was ultimately able to reclaim his positive outlook, and shared advice from his late father-in-law.
"He would always say, no matter what was going on, 'It gets better, kiddo. It gets better. The last thing you run out of is the future,'" he said. "He lived every day with gratitude and I realized, if there's gratitude, then you have sustained optimism."