The human eye has its limits. For motion, the eye can detect variation in approximately 50 frames per second.
The camera is like an eye. It lets light in. The camera-eye needs light to perceive an image. Film slips through behind the eye and light activates chemicals on the celluloid, which makes an image semi-permanent. We give to the camera a kind of memory, unstabilized, unlimited.
The camera projector is the opposite of an eye. It puts the memory out into the world. It creates light.
The projector works at near-capacity for human eye perception. It runs at 24 frames per second. Two little panels slide up and down to block out the light, reveal the light, block out the light, as each frame appears. (clickclickclickclickclickclick, etc) This light-no light-light-no light-light redundancy is what creates the illusion of movement. Movement in cinema does not exist. Only the film moves. Movement is interpellated by the small differences between frames.
Each frame is lit twice by the projector bulb, revealed twice through the snapping panels. This, ultimately, implies 48 frames per second. Why we don’t get angry that the movement is claymation-like, or stilted, or phony. We interpellate effectively at this rate. The magic of motion is nothing more than clever illusion.
In other words, watching a projected film you watch every frame twice. In essence, you see the whole film twice.
All in the name of fluid movement.