The humans share the same paper doll animation set, switching each body part frame accordingly. The zombies are all manually animated because of the varying sizes and body parts.
Creating a character at a three-quarters view is fairly straight forward. Given time, I think anyone can do it. If using Flash, just sketch with the brush tool, erasing as often as drawing new lines. Use Google Images for reference material. The head by far is the most important part. Be very sloppy and care free, as the purpose of the lines is to create the shapes and proportions. It should "feel" more like sculpting rather than drawing.
Occasionally, make a duplicate and then continue the process. Each iteration will bring you closer to the desired result, as you add things that complement the character, while removing features that hinder it. For example, the mustache, nurse hat, and cigar remained.
Continue until you end up with something good enough to finalize, by tracing over and splotching in shadows and highlights using the "Paint Inside" brush tool.
The process works well for me. It takes about one to four hours per character. It reserves ample time to dedicate to the many other important aspects of the game.