Secondary Action

This is an action which enriches and supports the primary action, adding more movement to the scene. However, this movement is intended to go unnoticed, and should not conflict or compete with the primary action of a scene. An example would be Luxo Jnr., by Pixar in 1984, where the lamp shade is jumping across the screen as the primary action, and then the wire trailing it as the secondary action.

Squash and Stretch

This is a useful way in animation to give the illusion of volume and weight when a character or an object goes through an action, such as facial expressions, exaggerated movements or a ball bouncing. Depending on the reaction that the animator desires the squash and stretch can vary, either very realistically or very over the top. For example in Fig.1 we can see the squashed and stretched forms of basic shapes, and in Fig.2 the forms of some objects. Also the video shows some more examples of squash and stretch.

Fig.1

Fig.2

Video

Exaggeration

Exaggeration is where the character's actions are expressed over the top and blown out of proportion. Exaggeration is usually used for comic relief or to show the weight of objects, regardless, in most cases exaggertion does not follow the rules of physical shape. Exaggeration helps to make animations less realistic and more exciting and interesting. However, with the use of exaggeration there needs to be a certain level of restraint, so that it the concept isn't confusing or overawing the audiences.
In (fig 1) the expression of the dog is quite exaggerated along with teh arms and hands, where they appear to be much bigger. The facial expression and arms help to exaggerate the emotion of the character.

(fig 1 )https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEij8RDUSgTzAEf9LBKzhVqxqI7FM7v2K5sCTSB_DnSub9xUWd75ABncduN-eQoTAYCmx9cOacjHJePKcmaT3ji-hCctpelYDRnhrtsePAEaZbI9IILdSmsw1kUUemffMk1Iw_6srs649nc/s400/corny1.jpg

Anticipation

Anticipation is when a character prepares to express an action, whether its preparing to run, or to catch a ball. Anticipation is vey important especially in helping the characters look realistic rather then jumping into an action instantaneously. Every animation should be applying this rule to all their characters, particularly if the animation is strifing for realism. Anticipation is also very useful to direct the audiences attention, as the picture below suggests, as Wile E Coyote prepares to run the audience would know he is about to run to the left because of the fact he is facing left. However, this can also help to create a comical effect, for example if the audiences expect a common action to occur from a character due to their anticipation, something entirely different happens.

An example can be Wile E Coyote, from looney tunes, where he would go into a pose getting ready to sprint:

http://www.officialpsds.com/images/thumbs/Road-Runner--Wile-E-Coyote-psd36956.png

Arcs

Arcs in animation is extensively used. Arcs allow the animator to plan out smooth, flowing paths of motions for objects, creating a natural feel to the animation (1). These arcs don't necessarily need to be paths of semi circles they are simply curved paths, for the objects as you can see in diagram (2).

(1) http://www.aneurismic.com/video/ballBounce_sketchbook.png

(2) http://www.angryanimator.com/tut/pic/002_walkcycle/wlk18.gif