Daniel Caylor found this link to Pixar Research Library which is I think their research papers, mostly fancy TD programmy stuff, but the article on Making Rats Appealing I felt was useful to animator's/ character designers.
1. low center of gravity/ flour sack shape & simple silhouette for old school classic Disney soft friendly character shape
2. made mouth muzzle really malleable so it can tie in with eye deformations, making the whole head feel connected and maintaining a sense of movement of the mouth even if the nose hides it (nose kept hardish to preserve skeleton) (look at the picture how the mouth points to the eye)
3. teeth lip controls to control composition of teeth "cradled in lips" or " sticking out ratlike"
4. deformable controls to allow animator to nail nice compositional silhouette's