The likelihood of deviation from the frame rate norm of 16fps during shooting was so great that the projectors of early cinema were usually fitted with variable speed dials which allowed projectionists to adjust the speed at which the film passed through the gate of the projector. By this means, they could ensure that the speed of the projector matched the speed at which the film had been shot in such a way as to produce normal speed of movement on the screen.
However, they would often intentionally set the projector at a speed that was at variance with the camera speed. If they wanted to speed the action up, say for a Chaplin slapstick comedy, they would increase the speed of the projector. For a romantic love scene on the other hand, they could slow it down a touch. Some directors would even stipulate different projection speeds for different reels of the same film depending on the subject-matter.
At other times, projectionists would speed a film up for the banal reason that they knew that it had to fit into a particular time-slot in the cinema programme. Making these speed adjustments was regarded as one of the most important skills that a good projectionist should have.