[If you haven't already seen Fred Astaire's gravity-defying dance routine from the 1951 movie "Royal Wedding" posted yesterday, be sure to check it out first.]
In this video, animator Galen Fott analyzes the number and shows how the secret was a hotel room set that could be rotated in a massive steel cage. Furniture was bolted to the floor and the camera and cameraman were strapped in and rotated with the set. Astaire, meanwhile, did his best to make it all seem perfectly natural that a human could dance on the walls and the ceiling of a room.
One of my favorite moments in the routine is when Astaire picks up (picks down?) the framed photo off the desk while he's dancing on the ceiling and then rotates the photo so he's looking at it right-side-up. From the accounts of the set crew, Fott explains that magnets were used to hold the frame in place while the set moved.