Squash & Stretch
The most important principle to animation is "Squash & Stretch", which is basically achieving a sense of weight and flexibility in drawn objects. This rule can be applied to simple objects, such as a bouncing ball, or objects of more complexity, like that of a human face. When this rule is taken to the extreme, by having a figure squashed or stretched to an exaggerated degree, we can create a comical effect.
In realistic animation, the most important aspect of this principle is that an object's volume does not change when squashed or stretched. Basically, if the length of a ball is stretched vertically, its width needs to contract correspondingly.
Timing to two different concepts: physical and theatrical timing. It is of vital importance to the storytelling of the animation, that the timing is done correctly. In terms of physical timing, by doing it correctly an object appears to obey the laws of physics. For example, an object's weight ultimately decides how it will react to an external force, such as a push.
Theatrical timing is less technical, and is developed through experience. It can be purely comic timing, or can be used to convey deep emotions. It can also be used as a device to convey aspects of a character's personality.
Anticipation is used to prepare the audience for an upcoming action, and to make actions in animation appear more realistic. Examples of this include a dancer jumping off the floor having to bend his knees first or a golfer making a swing having to swing the club backwards first. Additionally, this technique can also be used for less physical actions, such as a character looking off-screen to anticipate someone's arrival.
The purpose of this principle is to direct the audience's attention towards, and to make clear the most important elements in a scene. It is defined as "the presentation of any idea so that it is completely and unmistakably clear." This “presentation” can be done through various means, such as in the placement of a character in the frame, the use of light and shadow, and the angle and position of the camera. The heart of this principle is in keeping focus on what is relevant, and avoiding unnecessary details.
Follow Through and Overlapping Action
These techniques help to make movement more realistic, and create the illusion that the characters follow the laws of physics. "Follow Through" basically means that separate parts of a characters body will continue moving after they have stopped. "Overlapping Action" is when parts of a characters body move at different rates (an arm moving to different timing to the head and.) A third, lesser known, technique is "Drag", where a character starts to move and parts of him take a few frames to catch up. These parts are usually inanimate objects such as clothing or parts of the body, such as arms or hair.
On the human body, the torso is the core, with arms, legs, head and hair appendices that normally follow the torso's movement. Exaggerated use of the technique can produce a comical effect, while more realistic animation must time the actions exactly, to produce a convincing result.
Straight Ahead Action and Pose to Pose
These are two different approaches to the drawing process. "Straight ahead action" means drawing out a scene frame by frame from beginning to end, whilst "Pose to pose" refers to starting with drawing a few, key frames, and then filling in the spaces later. "Straight ahead action" results in a more graceful, dynamic illusion of movement, and is much better for producing realistic action. However, it is hard to maintain proportions, and to create precise, believable poses along the way. "Pose to pose" works much better for this, where composition and relation to the surroundings are of greater importance. Very often, animators use a combination of the two techniques.
Slow in and Slow out
The movement of the human body, and most other objects, needs time to accelerate and slow down. An animation looks far more realistic if it has more frames near the beginning and end of a movement, and fewer in the middle. This principle goes for characters moving between two extreme poses, such as sitting down and standing up, but also for inanimate, moving objects, such as a bouncing ball.
Most human and animal actions take place along an arched course, and for greater realism, animation should reproduce these movements. This can apply to a limb moving by rotating a joint, or a thrown object. The exception is mechanical movement, which typically moves in straight lines but which is never seen in the natural world.
Exaggeration is an effect that is especially useful for animation, as perfect imitation of reality can look still and boring in cartoons. The level of exaggeration depends completely on whether the animator seeks realism or a particular style. The traditional definition of exaggeration, which was employed by Disney, was to remain true to reality, just presenting it in a wilder, more extreme form. Additionally, other forms of exaggeration can involve the supernatural or surreal, alterations in the physical features of a character, or elements in the storyline itself. It is also important to restrain yourself somewhat when using exaggeration; if a scene contains several elements, there should be a balance in how those elements are exaggerated in relation to each other, to avoid confusing or overawing the viewer.
Adding secondary actions to the main action gives a scene more life, and can help to support the main action. A person walking can simultaneously swing his arms or keep them in his pockets, he can speak or whistle, or he can express emotions through facial expressions. The significant thing about secondary actions is that they give emphasis to, rather than take attention away from the main action.
Appeal in a cartoon character relates directly to what would be called charisma within an actor. A character who is appealing does not necessarily have to be sympathetic — villains or monsters can also be appealing. One such example can be found in the Disney/Pixar movie Monsters. Inc which is brimming with monstrous characters. The important thing is that the viewer feels that the character is real and is interesting. There are several tricks for making a character connect better with the audience; for likable characters a symmetrical or particularly baby-like face tends to be effective.