ET caught up with the 31-year-old actress at the premiere of her latest film, Cover Versions, in Los Angeles on Monday, where she told a story about randomly meeting the 33-year-old royal while vacationing in Miami with friends back in 2014.
"I was just there with some of my girlfriends, we had just finished shooting [Arrow], we were on vacation," Cassidy, who is the daughter of Sherry Williams and the late David Cassidy, explained to ET's Keltie Knight. "He had come with some of his friends ... one of his friends, I think, was getting married, so he was a part of the bachelor party. They were sorta on a bachelor weekend, and they were, like, 'Hey. You guys should come out with us.'"
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Cassidy says she was with two of her closest friends, who were both married, and although she's now happily engaged to Matthew Rodgers, she says she was single at the time. Prince Harry had just broken up with his ex-girlfriend, Cressida Bonas.
"[The bachelor party guys] were like, 'You guys should come out with us tonight.' I was like, 'Uh, no. With all due respect, Prince Harry, that's the last thing I'm gonna do -- be photographed," Cassidy continued. "Not that I have anything against him -- he's wonderful and lovely -- but, you know, I've also been 'on my own merit' type of girl ... not necessarily just the daughter of [David] or 'Harry's girl.' I have my own identity."
Despite turning down Prince Harry, who is marrying former Suits star Meghan Markle on May 19, Cassidy says he couldn't have been any kinder to her and her friends that weekend.
"He's the most loveliest, kindest person, honestly," she gushed. "And also, the fact that my girlfriends and I were like, 'Thank you so much, but no thank you, we're not gonna party with you and your friends tonight' -- he was like, we saw him the next day at the pool and [he was] respect[ful]. Like, 'You guys are cool. OK, we get it.' And I was like, 'Sorry!'"
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Cover Versions is available on digital on April 10, just over a month before Harry and Meghan say "I do" at St. George's Chapel. Obviously, we had to ask Cassidy if she'll be tuning in for what's sure to be the biggest event of the year.
"No," she joked. "But my mom is very into it. So, I'm sure I'll hear about it! Congratulations to them, that's awesome."
Be sure to keep up with all of ET's coverage of the royal family as we count down to May 19.