How do animators avoid drawing every one of 24 frames a second?

Animators avoid it by drawing only the poses that matter, the key frames, and letting someone else fill in the gaps. A key frame is a drawing that defines the starting or ending point of a smooth transition. A sequence of them defines which movement you will see, and where they sit in time defines how fast it goes.

Two or three key frames per second would not look like movement at all, so the frames in between are filled with inbetweens. In a traditional studio, a primary artist draws the key frames. Once the rough animation is approved, assistants clean it up and add the inbetweening. Dick Huemer developed this system in the 1920s, and Art Davis is said to have been the first inbetweener.

Even the gaps do not always need filling at film speed. The key animator typically does not draw all 24 frames needed for one second of film. Only very fast movements need 24 drawings per second, which animators call working on ones. Most movements can be done with 12 drawings per second, called working on twos, drawing one out of every two frames. This is not a recent shortcut: Fantasmagorie, from 1908 and widely considered the first fully animated movie, was animated on twos.

There is a floor, though. When there are too few inbetweens, such as four drawings per second, the animation may lose the illusion of movement altogether.