I have sometimes wondered if a subliminal image technique was introduced into old movies. I remember watching some Fred Astaire dancing routines and wondered if some of the optimum composition frames of the dance in motion were copied and repeated for a slightly longer duration (perhaps ½ a second) to give that wonderful graphic sense to the choreography. Perhaps Im just being cynical, or did anybody else notice this?
If you are not watching the movie at the cinema, you are likely seeing the '2 to 3 pulldown' effect. Every fourth frame the motion seem to stop. Watch in slow motion and see. The rhythm of the dance probably lined up with the 2:3 pulldown.
IIRC dance sequences were shot at differing frame rates dependent on the pace, background, and other variables. I think there is some mention of it in a Gene Kelly biography. He was a meticulous technician.
I think the definitive movie using inserted images into frames was the Exorcist. They also threw in techniques like squealing pigs in the soundtrack as well as lowering the sound and throwing out the focus slightly, just before something scary happened so you would draw in to hear and see, just before they sprang it on you, and scared the shit out of you.