I need to ask about drawing animal cartoon character with human characteristic features, to make it clear let's take a look at that picture

http://store3.up-00.com/Jun11/GZU08461.jpg 

There are a lot of face features that related between the actors and the cartoon character The question is how to do that?? Are there rules to draw like that?? any one know a reference to study that ??

hmm.. that is actually quite interesting.

I think it would be best to first draw the animal, and then look at the person it is based off of, modifying the face to resemble them more.