Earlier this year, I took a class taught by Stephen Silver and one thing he had us do a few times (primarily during the first few weeks of the course) was starting our drawings with an arrangement of three basic shapes: the circle, the the quadrilateral (most people would just say "the square") and the triangle. By varying the proportions and placement of these three shapes, you can come up with many different characters.
That is what I have done today. I have created three characters, each made up of each of the three basic shapes. Each shape varies in size in relation to the other two (e.g. the triangle will be noticeably smaller or larger than the square and circle), not only within the same drawing but in relation to the other two drawings as well. I admit the quadrilaterals in the drawings for the muscle man and short fellow are a bit too similar in size and shape for my taste, but at least I had the sense to orient them differently (vertically in the drawing on the left and horizontally in the middle one).
The placement of the shapes is different in each drawing as well.
The following image provides an overlay that shows the simple shapes I used when first constructing these characters.