Michael J. Fox told Jimmy Fallon how being seated next to Princess Diana at the “Back to the Future” world premiere ended up being a “nightmare” during an appearance on “The Tonight Show” Wednesday.

Fox, 59, who has been busy promoting his new book No Time Like the Future: An Optimist Considers Mortality, said of sitting next to the royal at the 1985 event: “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.”

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“The movie started and also I had to go pee,” Fox went on.

“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.”

The actor continued, “So it was just agony, it could have been the greatest night of my life but it was just a nightmare. A pee-holding nightmare.”

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As Fallon asked whether Diana, who tragically died in a car crash in Paris at age 36, enjoyed the movie, Fox replied: “She seemed to laugh a couple times.”

The interview comes as the star revealed in his new memoir that he’s retiring from acting again due to declining health.

Fox was diagnosed with early-onset Parkinson’s disease in 1991 at age 29. He went public with the diagnosis in 1998, becoming a strong advocate for Parkinson’s research through the Michael J. Fox Foundation.