Inside Joseph Gordon-Levitt's Secret 'Glass Onion' Cameo as the 'Hourly Dong' (Exclusive)

Rian Johnson landed a truly star-studded cast for Glass Onion: A Knives Out Mystery -- but there's one celeb you might have missed!

ET's Ash Crossan sat down with the director to discuss Daniel Craig's latest foray as Detective Benoit Blanc, but also got all the scoop on the film's "hourly dong" jingle, which was voiced by none other than Joseph Gordon-Levitt.

"He's in an episode of [Poker Face], this TV show I'm doing with Natasha Lyonne, Joe is the guest star of one of those episodes," Johnson said of how the cameo came to be. "I just said, you have to trust me, say the word dong really loudly."

So what is the hourly dong, exactly? 

"There's a thing on the island where every hour, you hear, 'Dong!' and it's the hourly dong, and goes off every hour," Johnson explained. "That's a reference to one of my favorite mystery movies, Evil Under the Sun, there's a big [plot point] in it with the 'noonday gun,' and they shoot a canon off at noon every day."

In Glass Onion, however, instead of a cannon, it's a "dong." Only that's not quite what it was supposed to be, Johnson revealed.

"In the script, it's written as, I think, the 'hourly gong,'" he recalled. "One time, when everybody was saying it, [Joseph] said it wrong and said the 'hourly dong.' And I said, 'That's it, it's the hourly dong now.'"

The director also opened up about two more meaningful cameos that he said were "incredible" to cast -- Angela Lansbury and Stephen Sondheim. The two share just a small scene in Glass Onion, but it ended up being the final on-screen role for both. Lansbury died in her sleep on Oct. 11 at age 96, while Sondheim died at 91 on Nov. 26, 2021.

"I don't want to blow it up too much, it's just one little fun moment that they have," Johnson said of the scene. "But what it meant for me, besides just how special it is to have both of them in the movie, it meant that I just got to spend 10 minutes with each of them to film this little moment."

"Getting to meet them, getting to tell Angela Lansbury what her work meant to me, telling her about watching the filmed version of Sweeney Todd that was on cable when I was a kid and how that really kind of started me loving musical theater... It felt like a very special privilege to get to do that."

Glass Onion: A Knives Out Mystery is now playing in limited theaters before streaming on Netflix on Dec. 23.

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