There are many principles of animation that have been used and perfected over the years to help make animated imagery more natural and believable. The majority of the principles are to make the imagery more fluid and less rigid. This is one of the main things that stands out as glaringly unnatural, the movement of the characters and the world they inhabit. Weight and momentum are two things that plague animators in all mediums and has done for years. It is infamous within the industry and can separate a piece from being good or exceptional. It is obvious in films like Transformers and Pacific Rim. Everything has weight and reacts effectively. It is a trait that is essential when dealing with anything that is meant to be photo real or composited in amongst live action footage. Below I will detail a few of the principles for more simplistic animation that I think will appeal to me and the style of design I want to try and create.
Squash & Stretch:
This is to do with movement and weight. When something moves it will thin out and create the idea of stretching due to motion. Also when things come to a sudden stop they will squash into a narrower shape to exaggerate the idea of stopping suddenly. This is used because it is very difficult to accurately draw what would happen to a real person if similar forces were applied. It would be exceptionally time consuming and very, very difficult. It is easier to cross over into the extreme and exaggerate it. This can be used to comic effect or used to a slightly lesser extent to merely make the object appear more natural and breath life into it.
This is the act of positioning characters and objects in a way that makes sense. It is near identical to "blocking". This is a principle in the film and theatre industry. This is where you position the actors on stage and almost choreograph where they will be at each moment and beat. This creates a predictable pattern for the cameras or audience to follow and is very important. It also helps the actors to anticipate certain actions and keep themselves in a good position. It can also be used past the mere function to imply moods or importance within the characters. It is used to make sure characters face the camera/audience when required and, particularly in film and animation where there can be multiple cuts and changes in perspective, keep the camera in a reliable position that the audience can recognise and understand.
Followthrough & Overlapping Action:
These are two fairly similar principles. Both are to create more realistic and natural movement. Followthrough action is similar to the squash and stretch technique detailed earlier. It is where a certain part of the body carries on moving even after the character has stopped. For example hair sways after the character has finished turning their head. A dress flows forward after the character has finished running. It ensures that it looks as though there is gravity and dynamism to the scene and again helps to breath life into the characters and their surroundings. It can also be used to an exaggerated extent to create a comic effect. This is visible in films by "egoraptor"on YouTube whereby characters body parts flail around crazily giving the impression that the character has limited control over his body. Overlapping is similar and means that multiple actions are taking place at once. For example an arm moves for longer than a head. This again creates a more natural and imperfect animation that looks less mathematical and formulaic. It creates a more human character and helps to make sure that theres always something to look at and motion to the scene/character and the audience is not just watching a static drawn image.
This is again a principle to do with weight. Most natural objects tend to move in an arched trajectory. If something is thrown or becomes airborne without lift it will fall back down. This gives the scene gravity and a sense of grounded realism. It is quite a simple, self explanatory rule that basically means that nothing in the scene defies gravity or becomes glaringly obvious as broken or unnatural.
This is similar to other principles previously discussed however, this is (surprisingly) more subtle. It is more to do with facial animation and other such subtleties. realistic and plain animation looks dull and lifeless in animation. The extent of exaggeration can vary depending on what look and feel the animator is going for. It can merely exaggerate a facial expression to make it more recognisable to the audience or it can massively morph the face into a completely unnatural expression for comic effect.
This is very similar to the idea of overlapping action. It is adding other forms of animation into the scene. A curtain blowing in the wind perhaps or a fan rotating. Anything at all into the background. This gives the scene in it's entirety life and depth. Nothing is notably static and lifeless there are subtle hints to other occurrences. This is quite important and usually forgotten where animators will just draw a static background but when done well it can be immensely powerful and can even be used to tell other miniature stories or tell side gags.
These are just a few of, in my view, the most important principles at the level of animation I am to be attempting. They all help to just breath life into the scene and engage the audience. Most of them appeal to character animation and frame by frame/stop motion animation however, I feel they are applicable to a wider array of animation types.