Theory of Animation

Animating things (creatures or objects) in a communicative or realistic manner can be challenging, but there are a number of tricks that can be useful.

Attraction Tricks

These tricks aim to attract the viewer's attention so that he/she does not miss something. This involves giving the viewer a hint that something is about to happen and where so that the user is looking in the right direction when the action itself takes ploace. Examples include an object blinking, rocking back and forth, backing up a little, or hesitating a momement before the action takes place.

Timing Tricks

Sometimes it can be appropriate to use non-realistic timings in order to achieve the desired communicative effect. For example, you can slow the speed down or speed it up, zoom. In some cases it can also be appropriate to use a logaritithmic scale rather than a linear one (e.g. if flyting towards objects that are vastly different in size).
You can make your animations seem more alive by making several things occur simultaneously. For example, a humanoid that is standing still might be seen to be breathing, but also blink or look around occasionally rather than look like a statue.

Motion enhancement Tricks

Sometimes, it is necessary to make it more obvious that a motion is taking place. For example a rolling ball may need some markings on it so that the user can clearly see that it is rotating as it moves. The same is true of smooth spinning objects, where it can be appropriate to add some marks. For real-time interactive 3D graphics, the small number of polygons and shading with highlights can give a good impression of motion without additional enhancement.

Other tricks

Other things that you should think about when designing an animation are conveying the weight and size of an object so that it's movements appear to be realistic. Also think about whether an object is soft or hard. A soft bouncy ball will distort as it bounces while a hard object that collides with something may simply vibrate or distort the object it hits.
If you are telling a story with animation (interactive or otherwise) then timing is very important. The viewer should anticipate actions, see them, and react to them, but this should not happen to quickly (or you'll lose the viewer) or too slowly (or the viewer will get bored).