While appearing on "Watch What Happens Live with Andy Cohen" on Sunday, Hickey, 57, was asked by a viewer if he's ever been starstruck by a celebrity who was in the audience while he was performing on stage.
Responding to the fan, the "In Treatment" star shared "a version" of a time he was starstruck, recalling an interesting story of when Madonna came to watch "Cabaret" many years ago.
Back in 1998, Hickey starred in a revival of the musical alongside "the great Natasha Richardson" and Alan Cumming.
"One night in the audience, Madonnawas there," Hickey explained. "And we were all so excited that she was there. And there were lights on the audience. Madonna proceeded to sit and braid her hair through the entire performance."
Although Hickey said he "didn't care one way or another," he said that it angered Richardson.
"Our leading lady, Natasha Richardson, came backstage with steam coming out of her ears. 'How dare she?,'" Hickey claimed, adding that Richardson was "absolutely right."
"If you're gonna come to the theater, don't sit and braid your hair," he said. "Have some manners, especially if you know all eyes are on you. So, I guess that was a version of being starstruck -- but not in the right way."
Richardson tragically passed away at the age of 45, after she suffered a head injury in a ski accident in 2009.
Meanwhile, when "WWHL" host Andy Cohen asked Hickey if he was "clocking" Madonna during the performance, he said he wasn't sure because he sometimes gets tunnel vision while on stage.
"Especially if someone famous is out there or somebody I respect a lot," he added.
Hickey revealed that the one and only Meryl Streep has been "in quite a few" of his audiences when he was on stage.
"I did a play with her daughter in summer stock," he recalled, "and my mom was coming to the show the same day but I didn't know that Meryl was coming and Mamie [Gummer] said to me, 'Oh my mom's out there today.' And I go, 'So is mine!'"
"It just completely freaked me out that Meryl was out there," Hickey said, before adding, "Meryl is great at backstage though. Meryl is one of those people who knows what power she has and she gives it all in a positive, wonderful way to other actors."
Cohen and Hickey then recalled a story of when they once played charades with Streep at Liam Neeson and Richardson's house.
See what they revealed Streep said of Cohen's charades skills in the full clip, above!