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How do key animators and in-betweeners divide the work of creating animation?

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Animated scene from The Wild Robot showing a round-eyed robot with a fox perched on its shoulder in a forest clearing. — Animation Scene from the 2024 science fiction animated film *The Wild Robot*, directed by Chris Sanders.

In the creation of animation, the labor is divided between key animators and assistant animators, often referred to as in-betweeners, to ensure fluid movement and characterization.1

Key Animators: These senior artists are responsible for drawing the "salients" or highlights of a movement. They create the primary frames—perhaps five or more out of the 24 required per second—that define the character's personality and the specific rhythm of the action.1
In-betweeners (Assistant Animators): These artists are tasked with closing the gaps between the key drawings. They complete the intermediate frames necessary to create a smooth, continuous sequence of motion.1

This division of labor is essential for bringing figures to life, with the workload intensity often scaling based on the size of the animation unit. The entire process is guided by a workbook or time chart, which acts like a musical score to ensure that the animators' drawings are precisely timed frame by frame.12