All the Disney Feature Animation interns gathered around Sam Ewing‘s animation desk to watch him point out the mechanics of inbetweening. In an animated scene you often have tow key drawings which show the extremes of a motion.
Sam has Mickey on his desk, so I will use him to explain. If Mickey throws a baseball you will have the arm pulled back anticipating the throw and then you will have the hand extended forward at eh moment the ball is released. The inbetweens fill in the gap between the tow extremes.There are 24 frames of film for every second so it take 24 drawings for a one second movement. It is often possible to get away with 12 drawings and holding each drawing for two frames each. In any scene more that 3/4 of the drawings are inbetweens. in a 2 hour movie that means there are about 172,800 animated drawings that need to be done. That is why a small army of artists are needed to complete an animated film.
After work I walked around the empty MGM theme park with several other animators. The park was closed but the music was still playing. It felt eerie. The whole Disney experience was kind of eerie.