It’s important to talk about the principles of animation before going into the subject itself. These principles were introduced nearly 35 years ago in a book by two Disney animators, however they do go into the company’s animation styles and techniques spanning back to the 1930s.
Squash and stretch – This is about physics in a drawn character, giving the character a sense of weight and flexibility by changing shape. This grounds it, although it can very much be combined with exaggeration, as was well-known in Disney.
Anticipation – This principle is necessary in animation, where it’s used to prepare the audience for an action that is about to happen in the scene. This can be pulling back before throwing a punch or squatting slightly before jumping in the air.
Staging – Much like in theatre, the scene itself is a character that needs to be thought about. You need to direct the audience’s attention to certain parts and certain times, so the audience knows what’s important, and this is what staging is all about.
Straight ahead action and pose to pose – These have to do with drawing processes, which either can be directly liner (frame by frame) or drawing key frames and then filling in the intervals.
Follow through and overlapping action – These principles are closely related and are about rending movement more realistically, to give the sense that the animation is following the laws of physics.
Slow in and slow out – These address real-life movements, where items/people accelerate and slow down towards the extremes of their motions. A swinging arm will slow before it reaches its peak, for example.
Arc – Most natural actions tend to follow an arched trajectory, so this is used to add more realism (again). Arms and legs will move in arcs, much like a blade will be swung by a character on an arched trajectory towards its foe.
Secondary action – These are used to give the main scene more life, so while the focus might be of a man walking, he may also be juggling some change, swinging his arms, whistling a jaunty tune, etc. to inject more life to the scenario.
Timing – In this sense, timing is about the frames used per given action (so the speed of the action), rather than the comedic timing in a cartoon. This is all on a purely physical and real-life level.
Exaggeration – Using realism in animations isn’t always right for some particular styles, so exaggerating movements, looks, physics, etc. may work better for the situation. Disney did use this in moderation, as do many other animation companies. It’s about finding the middle-ground between realism and the style.
Solid drawing – This principle is fairly straight-forward, as it should be applied to most kinds of drawing. The animation will take on 3D weight, volume, light, shadows, all of these grounding it more in realism.
Appeal – This is the appeal of the character; does it satisfy the part its trying to play in the animation. This can mean good guys as well as bad guys. It boils down to whether the character is plausible in real life.