In the early days of animation, most of the motion depicted was stiff and mechanical. Aiming to develop a more realistic look, animators in the Golden Age honed their craft and came up with innovative techniques to make their animations more fluid and organic. In 1981, Disney animators Ollie Johnston and Frank Thomas codified these techniques as the 12 basic principles of animation. These principles were developed by studying the way people and things move in real life, giving them a basis in physical reality.

Though by no means required, applying the 12 principles can bring an animation to life by giving it weight, cohesion, and clarity.

Squash and stretch

Different things in the real world have different levels of flexibility. Flexible objects will stretch when moving and squash when stopping. Applying squash and stretch to your animation can give it a sense of life and organicness.

Anticipation

An action consists of three parts: anticipation before the action begins, the action itself, and termination when the action ends. Showing the anticipation phase in your animation will give it a sense of weight.

Staging

Staging means presenting your idea in a way that is visually clear. A good animation should not be obscured by excessive detail or too many things happening at once.

Straight ahead and pose-to-pose

When animating a scene, there tend to be two ways to go about it: straight ahead and pose-to-pose. Straight ahead animation is when the animator creates a scene one drawing after another without planning ahead, best utilized in frenetic and spontaneous moments.

On the other hand, pose-to-pose animation involves planning out beforehand the key poses of an animation and drawing the frames inbetween, good for slower scenes where maintaining control over timing and proportions is more important.

Follow through and overlapping action

Follow through and overlapping action refer to how parts of a character such as their limbs, hair, or clothing react when there's a change in their motion. When a character stops or changes direction, these parts should breifly continue in the direction of the original motion and "catch up" over the course of a few frames.

Slow-out and slow-in

When drawing the frames between two poses, there should be more frames at the beginning and end than there are in the middle. This will make motion feel softer and more natural as opposed to stiff.

Arcs

Actions in the real world tend to follow an arc, with the exception of mechanical devices. Incorporating motion arcs into your animation will make it much more expressive.

Secondary action

Secondary actions are actions that accompany and enhance the main action, such as a character swinging their arms while walking. Incorporating secondary actions will give a sense of complexity to your scene without being unnecessary or distracting.

Timing

Perhaps the most important principle, timing decides the feel of your animation to a great degree. More frames will make an animation feel slow and smooth while less frames will make actions feel quick and snappy.

Exaggeration

While you usually don't want your animation to be too overly distorted, applying some degree of exaggeration to a characters' actions will go a long way in making them more natural and appealing.

Solid drawing

Having a foundational understanding of form, weight, and 3D space is important if you want to make your animations appear lifelike and realistic.

Appeal

An appealing character is a character the audience enjoys watching. Audiences will like characters that are charming, simply designed, expressive, or compelling rather than dull, overly intricate in design, stiff, or forgettable.