There are some basic principles of animation
that every animator should become familiar with. These are not rules, but techniques. Even if you intend to go against these techniques, you should have an understanding of the principles and know why you are choosing to ignore them. Through an understanding of these essential principles, you can have a better idea of how viewers will react to your animation.

In essence, these principles are observations of the way we see the world and techniques to modify your viewers interpretaion of what they are seeing.

 

Squash and Stretch
One place we see "squash and stretch" is in the way our eyes interpret motion. As an object moves, our eyes do not see a rigid form. Rather, objects appear to "stretch" as they speed up and "squash" when they come to a sudden stop. Incorporating this into your animations will result in a much more natural looking motion. One way to think of "squash and stretch" is a bouncing ball. As a ball hits the floor, it appears to "squash" into the surface before bouncing back up. "Squash and stretch" is the easiest way to demonstrate the weight or personality of an object as it moves. Heavy objects tend to squash more than lighter objects. Sad objects tend to squash more than happy or energetic objects. This simple principle can be used to show personality in just about anything.

Anticipation
A wind-up before a baseball pitch, a character's eyes opening wide at the sight of something, or stepping back and pausing before lunging forward to push a heavy object. Anticipation is a way of letting the viewer know what is going to happen before it actually takes place. Just about every motion involves some sort of anticipation, and leaving anticipation out of your animation can result in overly sudden movements. From simple movements to the most complex, anticipation is important. Before turning to the side, a character will usually lead with her head. In addition to showing natural movement, anticipation can be used as a story-telling device, having a character react to something off-screen before the audience is actually shown the event. This prepares the viewers for what they are about to see and demonstrates the character's emotion and reaction to the event before it even occurs. You can tell the audience that there is something scary off-screen without ever showing what it is.

Staging
This is a principle which finds its roots in the theatre and is basically the placement of objects and characters in relation to eachother and to the camera. Proper staging is important to keep the viewer from getting confused and to clearly demonstrate the action which is taking place.

Follow-Through
Whereas anticipation precedes motion, follow-through is present after a motion. After throwing a ball, a pitcher's arm continues along its path and he makes a step forward to catch his balance. When a car comes to a stop, it rocks forward a bit, then settles back. Motion without follow-through can look very stiff and robotic. A little follow-through should be used with every motion, even if you are animating a stiff robot.

Overlapping Action
When catching a ball, your eyes follow it, your arms reach out, and your hands turn to catch the ball, all at the same time. This is an example of overlapping action. Showing a character's arms simply jut out and catch a ball is unnatural. Showing a character's head turn to look at something without his shoulders and torso turning as well is unrealistic. Most motion is accompanied by overlapping action. When you are creating a motion, be sure to look beyond the motion itself and see what other elements are involved in making that motion possible. Your head cannot turn 180 degrees, but you can look 180 degrees behind you with the help of overlapping action.

Slow In, Slow Out
This is the basic, but very important observation that objects do not simply move and stop in rigid increments. When you turn your head, it moves slowly for a moment before it can turn quickly. Before it comes to a stop, it slows down. This keeps our necks from breaking, and it keeps your animations looking smooth and natural. Jerky movements result from failing to "slow in" to motions and "slow out" from them.

Arc vs. Linear Motion
Another important observation is that very few beings in this world move in a linear fashion. You may think that as you reach down to pick up a ball, your hand is following a linear motion. In fact, it follows a slight curve. When you sit down, your body does not move straight towards the chair. It follows a curve as well. Incorporating and observing curves in your motions will result in a more natural appearance.

Secondary Action
Secondary action is just as it sounds- an action is taking place in addition to the main action. One example would be a character walking forward and simultaneously looking to his left at an attractive woman passing by. The primary action is walking. The secondary action is looking. Secondary action can be used to convey emotion and thought. When you are animating, do not limit yourself to: "Character does A, then B, then C, then D." Remember that the character can do "A, then B while doing C, then D". Complex characters sometimes require complex secondary actions.

Timing
Timing is at the root of all motion. As you walk, when does your right foot hit the ground in relation to your left? How long does it take a character to react to a gun shot? Does a happy character react differently than a sad one? Timing can be used to show emotion, character, and action itself. Timing can also be used to build suspense, cause panic, or relaxation. The human eye reacts strongly to the timing of events. Consider carefully when certain events should occur, and use timing to your advantage to convey character, emotion, and in storytelling.

Exaggeration
Exaggeration should be used to draw attention to the main motion in a scene. Although it may seem silly at times, exaggeration creates a more believable motion that the eye is drawn to. It is a common mistake to under-emphasize a motion. The result is tired-looking, lifeless characters. Try to keep your motions deliberate by exaggerating them a little and giving your characters life.

Knowledge of Technique
Simply put, to create a successuful animation, you need to know what you are doing. If you are drawing a traditional animation, you better know how to draw. If you are creating a computer animation, you must learn the software. You do not need to be a computer wiz to create a succeful animation, but you do need to have a grasp of the tools at hand.

Observation and Translation
A very important part of animating successfully is being able to observe the world around you and translate it into animating terms. Creating a character throwing a ball? Throw a ball yourself and observe what motions you make. Repeat about 100 times. Better yet, videotape yourself and play the tape back, fist looking at your arm, then at the other elements such as your torso and legs. By observing reality, you are far more likely to create a great-looking animation than if you just sit down and try to fake what it would look like.

Appeal
A successful animation is appealing to the viewer. Jerky motion can detract from an animation's appeal, as can poor exectution, a poor story, and lack of imagination. Try to create something original that the viewer will enjoy. Most of all, work on something that you are excited about. If you are excited about what you have created, the odds are that someone else will be excited by it as well.

Imagination
Without a little imagination, it would be impossible to bring all of these principles together and use them in a meaningful way. Imagination is essential to your success as an animator. Every motion that you make, use your imagination to explore how the motion can vary to show the image you are trying to create.

 

For another look at the principles of animation, see
Disney Animation: The Illusion of Life
by Frank Thomas and Ollie Johnston