So when I started working on Jack in mid-2003, I began by writing a bunch of episodes in a Creative Writing Workshop by Dawn Trook, a friend and a terrific writer. The visual character of Jack at that point was just a circle-head with ear-blobs and some scribbles for facial expressions. Those story sketches evolved with some cursory photo research into the fellow at right:

This worked well enough for a while, and I even finished 5 episodes with this character and similar proto-versions of Mel and the Kit Fox, each of which had their charm (more or less).

But then we went on a camping trip to Big Bend National Park and saw many, many jackrabbits hanging around our campsite on the desert floor.
I saw right away that THIS WOULD NOT DO.
Head shape: wrong.
Tail: too puffy.
Overall: not quite gangly enough.

Of course these were only a start, and at my wilfe Ellen's suggestion, I worked on getting a better sense of actual rabbit construction. Rather than starting with a round head and applying rabbit-esque attributes, I would instead go to the actual rabbit and add cartoon-esque attributes to that, as below:

But these drawings, while they felt more accurate, lacked almost all of the cuteness that came from the simplicity of the earlier versions. So I tried combining the two:

 

This was working much better for me and led to this sketch of the final version (with body), at right:

And there you have it. I'm interested to see how the characters continue to evolve through the actual strip production over time, but here you have a snapshot, anyway, of the process so far...