Twelve Principles of Animation
1. Squash and Stretch.
2. Anticipation. This is setting up the action before it happens, usually with a slight movement in the opposite direction to the main one.
3. Staging. This is related to the way the film as a whole is "shot," considering angles, framing, and scene length.
4. Straight-ahead Action and Pose-to-Pose. Straight-ahead action starts at one point and finishes at another in a single continuous movement, such as running, whereas pose-to-pose is a variety of actions in one scene requiring clearly delineated key frames to mark the action's extreme point. How the in-betweens are executed can alter the whole rhythm of the action.
5. Follow-through and Overlapping Action. Follow-through is the opposite of anticipation. When a character stops, certain parts remain in motion, such as hair or clothes. Overlapping action is when the follow-through of one action becomes the anticipation of the next one.
6. Slow In -- Slow Out. This means using more drawings at the beginning and end of an action and fewer in the middle. This creates a more lifelike feeling to the movement.
7. Arcs. These are used to describe natural movement. All actions create circular movements because they usually pivot around a central point, usually a joint. Arcs are also used to describe a line of action through a character.
8. Secondary Action is just that, another action that takes place at the same time as the main one. This may be something as simple as turning the head from side to side during a walk sequence.
9. Timing. This is something that can't be taught. In the same way that comedians who rely on it to get the most from their gags have to learn it through experience, you too will get it right only through practice. Timing is how you get characters to interact naturally. Timing also has to do with the technical side of deciding how many drawings are used to portray an action.
10. Exaggeration. This is the enhancement of a physical attribute or movement, but don't make the mistake of exaggerating the exaggeration.
11. Solid Drawing. This conveys a sense of three-dimensionality through linework, color, and shading.
12. Appeal This is giving personality to the characters you draw. If you can convey it without the sound track, you know you are on the right track.
These are not hard and fast rules, but they have been found to work since the early days of animation. Bear them in mind at the storyboard stage and your animation will definitely have more fluidity and believability.