Billie Eilish dances on the ceiling in her Saturday Night Debut performance. (Photo: SNL/ScreenCap)

Billie Eilish made her debut on the season premiere of “Saturday Night Live” with a snappy rendition of “Bad Guy” that saw her walking on the walls and ceiling of a room. But it’s all been done before… not that her fans would know.

Legendary movie star and dance phenom Fred Astaire created the signature “rotating room” dance for the 1951 movie “Royal Wedding.”

Eilish, who turns all of 18 in December, may or may not have known about Astaire’s movie, but many of her fans certainly didn’t.

“Throughout this whole performance the only that went through my head was one question, WHAT IS GOING ON,” wrote one fan in YouTube comments.

“Who else tried rotating their phone when she started climbing the wall lmao,” wrote another.

And in the penultimate comment: “You can’t deny she was meant to be onstage because this is one of the most creative and unique performances I’ve ever see on SNL,” gushed a third fan.

Yeah… ah no.

Last year, Ariana Grande created the same illusion in her “No Tears Left to Cry” video.

But the honor of being “unique” goes to Astaire.

Of course, no artist can dance on the ceiling. The illusion works by rotating the room along with the camera.

In “Royal Wedding,” the entire set rotated as Fred danced and made it all look so real while he sang the song “You’re All the World to Me.”

Director Stanley Donen filmed the scene.

The hotel room set was constructed inside a huge rotating steel cage, all the furniture was bolted down and the camera and cameraman were strapped down and traveled with the set. while Astaire danced away, always remaining upright as the room rotated around him.

In one gravity defying moment, Astaire pulls a photo down from the desk. Magnets kept it in place, according to Donen.

Throughout the number, the room rotates 360 degrees clockwise, then 270 degrees counterclockwise, then 270 degrees back clockwise,” according to Bigfott Productions, which breaks down the scene.

The Eilish dance scene takes place in a box painted to look like a room.

Grande used much more production value and likely some CGI to boot. Her scene is set in a three-dimensional hallway, as elaborate as Astaire’s set.

The idea occurred to Astaire back in the 1920s and was first mentioned by him in the MGM publicity publication Lion’s Roar in 1945, according to some accounts.

The film was one of the top box office hits of 1951, and rottentomatoes gives it a 91 rating.

A similar scene also appeared in “Euphoria,” HBO’s breakout hit this past summer and the 2010 movie “Inception.”

So, well, nothing new about it. But, hey, Billie looks pretty cool doing it.

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