The visual style of rendering, light, shadow, detail, texture, material, shape and form as expressed in an animated world (regardless of medium - 2d, CG, stop-mo, paper cut out, oil on glass, etc.) demands a harmonious matching in the design and motion of the characters that live within that animated world.
Always the one to try and boil things down into simple to digest nuggets, I've settled on the idea of calling this principle Visual Harmony. It's instinctually understood by just about everybody (even those not trained in animation), though I don't see a lot of discussion about it. Some cursory searches on the topic (or other similar ideas) yields very little. I'm not an art historian so those of you who know about such things are more than welcome to ed-yoo-kate me on any theses, papers or books written about the subject that I have missed. All I know is what I have discerned with my own eyes and this thing I've discerned is definitely present. Basically, visuals and motion and shapes and forms all desire to be in harmony. When they are not then a discernable (if not defineable) dischord occurs.
Over the years here I've talked about all manner of things related to how animated film and motion work together. Recently I've written about the general preference for CG films to engage in a kind of exaggerated naturalistic motion. Aside from some controversial suggestions about the future methods for best obtaining that style of motion, I want to point out that I'm not for or against the style itself. As people like to say these days "It is what it is." In fact if we look at the salient technical and visual developments in CG over the last 20 years we can pretty much conclude that exaggerated yet naturalistic motion style (and character design) pretty much has been demanded. Anything else would have been 'wrong'. The driving force was this unspoken, ill-defined concept of visual harmony. Once you ramp up one area of the visuals into a higher order of complexity (say, lighting or texturing) then all other areas need to rise accordingly to maintain that harmony.
From the very beginning the chief developments and advancements in Cg have been to recreate some aspect of nature. Today we like to remember Pixar's first short "Luxo Jr." for its quaint story telling, but those who go back a little further remember all the excitement over the chief technical accomplishment of that film- self shadowing. Technological advancement in reflecting some aspect of the world around us with varying degrees of versimilitude was Pixar's big selling point in those early short films, and still remains a stated goal with their current shorts. Thus as these visual abilities matured they demanded ever increasing refinement in all other visual elements in order to maintain that strong sense of visual harmony- including the motion. The motion simply must "match" the visuals. This is why old classic Golden Era cartoons from the 1940's can get away with motion styles that - if transliterated without interpretation- would look absolutely out of place in your typical, "hyper-real", textured, shaded, lit, detail oriented CG film. Folks have tried to marry the two with limited success. The reason for the lack of pleasing result is simple- the two styles are not harmonious. You can't put new wine into old wineskins. The stuff's gotta match. From the start CG has this domineering way of making everything look 'realish'. Even if you dial back the textures the very shading algorithms themselves create a kind of realish-ness. Early NPR efforts (non-photoreal rendering) rang a sour note because we had a type of simulated rough analog visual medium (oil paint, pastels, etc.) being employed in expressing very smooth animated motion on 1's on models that did not vary in volume or line width. The dis-harmony was very apparent, which is perhaps why NPR never quite caught on. When viewed as a still the visuals said "hand made". When seen in motion too many elements said "brought to you by a machine". The stuff just didn't fit together. So far the most successful adventures in defining different motion styles in CG have been done by reducing visual elements. Pocoyo removes everything but the characters and primary props. The reason this is successful is because the motion does not need to live in harmony with any other visuals. In musical terms they can just sing the melody because there are no other parts to be sung. It's an extremely clever solution. Another successful result was Marc Craste's Pica Towers from earlier this decade. By removing color and subtlety of shading (much of it is very stark with little gradiation in values) he simplified the visuals which allowed him to employ a more limited motion style. The result was immediately satisfying to behold. The harmony was a mystery to me back then and it is no less striking today.
For me this is more than an academic exercise. I want to make quality animated products, using CG, but I want to do it in a way that actually lets me make more films faster, in a more fun and interactive manner. But the fact is that the predominant CG feature film style (as first defined by Pixar and subsequently copied by everybody else) is orders of magnitude too complex to pull off with anything less than a veritable army of artists, animators, technicians and machines. The style complexity - reflected in its realishness- absolutely demands visual harmony on all levels. Drop the level of complexity in any area and you create a sort of "off-key" feel to the animated film. General audiences understand this. When the visual harmony is well done they immediately perceive the animated film as having 'quality'. In the past we've made the mistake in thinking that this complexity was itself quality. This is a false dichotomy which has been bashed into our heads for almost two decades now. There is such a thing as animation that lacks complexity or realism or literalism but yet still has a high degree of quality, simply because all the pieces 'fit'. So complexity or literalism is not the equivalent of quality- visual harmony is. If you want to make something of real quality, then make something harmonious. In this light many of the 'rules' and 'principles' that have lorded over animation fade into their proper place. The limits are removed and whole new possibilities emerge.