I remember buying a piece of original comic art by Buscema and on the back page it had a few doodles that he had done for his own amusement. I learned that he had a habit of making these drawings on the back of his comic pages, some were even better than what he had on the front.
There have been a couple of books put out on these sketches and a bunch of articles in magazines like Rough Stuff and Alter Ego I have them all as well as some originals. I poured over these to learn what I can and from this I developed a little exercise that I use to doodle. In Buscema's drawing you can trace the skull underneath. I found the following Buscema drawings on the internet and am using them here as an example.
On the right I traced the shape of the skull over the drawing to show the structure of the drawing. The more I looked as his drawings the more I kept seeing the skull underneath.
Bellow are two more examples of John Buscema's sketches.
These are some sketches I came up with based on the outline of the skull underneath.
Sometimes I would trace over a drawing I made of these outline's to see how many different face's I can come up with, other times I would just lightly indicate the skull and add the features on top of that.
I might ink them or color them or apply washes of gray.