Depending on the projector capabilities, they can be shown at a range of speeds - faster and slower than 24 fps. Many of the older silent movies were often cranked at a slower speed, even down to 18 fps.

For example I once watched the Cabinet of Dr Calgari at the 'standard' speed, then we set it slower, and all the 'jerkiness' disappeared, and the actions seemed at entirely normal speed (it was a 16mm print).

Most movies shown on TV are shown at a speed more compatible with the TV framing rate just by running the film faster. There used to be a complicated optical gadget for transforming the gate rate by screening the film at its correct speed onto a glass plate that was then 'filmed' by either another camera, or a TV camera. Was considered a hideously financially process though, so apparently fell into disuse.