Basic Principles of Animation

Squash and Stretch
Squashing and stretching gives the illusion of weight and volume to an object and character in an animation. The use of squash and stretch differs depending on the animation where in short style of picture, squash and stretch is broader but subtler in a feature. Squash and stretch is used in all kinds of character animation, from bouncing a ball to the weight of the person walking.
The diagram shows a ball bouncing where on the left the ball is the same size all the way through the animation but on the right it changes shape and size. Squashing and stretching the ball in this animation makes a huge difference as the animation looks more realistic.

Anticipation
This refers to the preparation the audience go through before the character is about to perform an action in the animation such as: starting to run, jumping or a change in facial expression.
 For example, when a character is about to jump, they bend their legs before leaping. The bending of legs is the anticipation movement before the jump is executed.
Almost all real action has anticipation. Feature animation is often less broad than short animation unless a scene requires it to develop a character's personality.

Staging
Staging refers to the pose or action that should clearly communicate to the audience the mood, attitude, the reaction or idea the character is trying to convey for the continuity of the story line. Using long, medium or close up shots effectively helps tell the story and gives the audience a better understanding of the character. Staging directs the audience's attention to the story or idea but too much action in a scene looks clustered and causes confusion.It's necessary to make sure the background in the scene isn't over powering the character so that the audience's attention stays on the character and not on the background.
The picture shows poor staging and good staging. The poor staging image doesn't show the character's faces as they are facing each other and this deprives the audience of their facial expressions which may be important in conveying the scene. Although, the 'good staging' image shows the characters faces and expressions which gives the audience a better understanding of the scene and characters.

Straight ahead and Pose to pose animation
Straight ahead animation starts at the first drawing and continues to drawing to the end of the scene where you can lose size, proportion and volume with this method. Many fast, action scenes are done this way as it has spontaneity and freshness. Pose to pose is more planned and charted with key drawings done at intervals throughout the scene and size, volumes and proportions are better controlled this way.

Follow through and overlapping action
Follow through is when the other parts of the character, (e.g. the tail, clothing, hair) catch up with the main body of the character. Overlapping acton is when the character changes direction while their clothes or hair continues forward. As the character is going in a new direction, their clothes will follow a few frames later. "Drag" would be when the character, for example, is running and their head, ears and upper body do not keep up with the main body, this action is done more subtly.

In this example, as the head goes up, the hair moves as well as part of the follow through action to make the animation look more realistic. The same happens when the head goes down, the hair moves along with it.


Slow-Out and Slow-In
The more drawings an action has, the slower the animation will be as there are more frames. On the other hand, the fewer frames there are the faster the animation will be. Slow-ins and slow-outs soften the animation/action making the scene seem more life-like.
The movement of the human body needs time to accelerate and slow down. All moving objects start slowly and finish slowly, therefore there are more frames at the beginning of the animation, fewer in the middle and more towards the end. This technique helps to recognise what the object if because if it went too fast, we would not be able to recognise it.


Arcs
Most action follow an arc or slightly circular path for greater realism in an animation. Arcs give an animation a more natural action and better flow. As an object's speed and momentum increase, arcs tend to flatten out but arc more as the object gets slower, as shown in the diagram.


As the swing is in the air and falling down, the movement is slow so there is more of an arc which straightens out as the swing goes faster.





Secondary Action
This adds and enriches the main action as well as adds more dimension to the character animation. For example, a man walking down a road will move their arms backwards and forwards to make the action of walking look more realistic.





Timing
More drawings between poses smooth out the action and make it look less jerky. Fewer drawings make the action look faster and more defined. A variety of slow and fast timing within a scene adds texture and interest to the movement. There is timing in the acting of a character to establish mood emotions and reaction to another character or situation.


Exaggeration
This is like a caricature of facial features, expressions, poses, attitudes and actions. The key is to take something and make it more extreme in order to give it more live. For example, exaggerating the proportion of a lamp to give the sense of a father and son.

Solid Drawing
The basic principles of drawing form, weight, volume solidity and the illusion of three dimension apply to animation, the same drawing. The principle of solid drawing means taking into account forms in three-dimensional space, giving them volume and weight.





Appeal
All characters have to have appeal whether they are heroic, villainous, comic or cute. Appeal corresponds to charisma in a character. Appeal includes easy to read design, clear drawing and personality development that will capture and involve the audience's interest. For example, for likeable characters, a symmetrical or  baby-like face tends to be effective. Like all forms of story telling, the feature has to appeal to the mind as well as to the eye.



source(s):
http://minyos.its.rmit.edu.au/aim/a_notes/anim_principles.html
http://ticktockcrocodile.blogspot.co.uk/2010/12/slow-in-and-slow-out.html
http://en.wikipedia.org/wiki/12_basic_principles_of_animation