The 73-year-old actor shared a memory of the late British royal during Monday's episode of Watch What Happens Live.
Crystal and Meg Ryan starred in When Harry Met Sally, a romantic comedy that opened in theaters in 1989. Diana attended the London premiere of the film and was seated next to Crystal and Ryan.
On WWHL, Crystal said the audience turned to see Diana's reaction to When Harry Met Sally's famous scene where Ryan's character fakes an orgasm.
"When the orgasm scene came on -- we were in the front row in the balcony, and everybody I could see in the orchestra below us turned their backs to the screen to see how she was going to react," Crystal said. "So there was hundreds of people just staring at us in the royal box."
"She started laughing so hard. She had one of those kind of laughs that if it was a date I don't think I'd see her again," he added of Diana. "But she was so wonderful and loved the movie and it was really a thrill to meet her."
"I happened to stand right near him when he was making his speech ... we look at each other and we just embrace and that was one of the really great moments for me," Crystal said.
Crystal will return as the voice of Mike Wazowski in the upcoming Disney+ animated series Monsters at Work.