Skill, practice, using live action references.
It is quite common with animation to film the same actions live, and then use those as a reference for the animation. There are videos on youtube of this being done by old school Disney animators.
I know that Aardman used to point a video camera in the same direction as the proper film camera. This would be shown on a small TV set so they could see the camera’s eye view of the scene. They would use a felt tip pen to track certain features of the scene (such as a character’s nose) to make sure the movements are fluid.
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