Michael J. Fox Details 'Nightmare' Scenario Watching 'Back to the Future' Next to Princess Diana

Michael J. Fox said it was a "nightmare" to sit next to Princess Diana during the 1985 London premiere of Back to the Future. The actor said he had to go to the bathroom during the movie, but he could not figure out how to get up from his seat with a member of the British royal family beside him. The 59-year-old Fox recently announced he was retiring from acting due to his declining health.

"She was sitting next to me. The lights go down and the movie starts, and I realize I am one fake yawn and an arm stretch away from being on a date with her, which is hilarious," Fox said during an appearance on The Tonight Show Starring Jimmy Fallon on Wednesday, reports Us Weekly. When the movie started, Fox realized he suddenly needed to go to the restroom. "So, for the rest of the movie, I'm sitting there, like, dying. I can't say anything to her and I can't walk away from her, because I can't turn my back on her," he said.

Fox said it was "just agony" during the screening. "It could have been the greatest night of my life, but it was just a nightmare. A pee-holding nightmare," he said. Fallon later asked if the late Diana liked Back to the Future. "She seemed to laugh a couple times," the actor said.

The Teen Wolf star has been busy lately with the release of his new memoir, No Time Live The Future: An Optimist Considers Mortality, which hits bookstores on Nov. 17. In the book, Fox revealed he is stepping away from acting as his battle with Parkinson's disease progresses. "There is a time for everything, and my time of putting in a 12-hour workday, and memorizing seven pages of dialogue, is best behind me," the beloved star wrote. "At least for now... I enter a second retirement. That could change because everything changes. But if this is the end of my acting career, so be it."

Fox was diagnosed with Parkinson's in 1991, shortly after he finished filming the Back to the Future sequels. In 2000, he retired from acting for the first time after he left Spin City. The actor eventually returned to television by making guest appearances on Rescue Me, Boston Legal, and The Good Wife. He also starred in a short-lived NBC sitcom called The Michael J. Fox Show. He made his most recent TV appearance on The Good Fight, reprising his Good Wife character, Louis Canning.

In a new interview with PEOPLE, Fox said his short-term memory is "shot," making it difficult for him to remember lines. He said the one thing he can still do well is write. "I'm down to this," he said. "My guitar playing is no good. My sketching is no good anymore, my dancing never was good, and acting is getting tougher to do. So it's down to writing. Luckily, I really enjoy it."