I recently had the opportunity to corrupt young minds at SCAD with a lecture on body mechanics and acting. To demonstrate a few basic principals I created this rig I call Generik. Here he is doing a basic "Neutral" walk:
Among humans, the biggest factor affecting walk is gender. Here I contrast an exaggerated feminine walk with an exaggerated masculine walk:
The second biggest factor in determining the style of walk is the body type of the individual. Here I contrast a "fat" vs. a "skinny" walk:
The third factor is mood. Individuals display mood in their posture. Here is an exaggerated "Happy" vs. "Sad" walk:
I created all of these cycles on animation layers in Maya 2009. You can copy animation layers to try out variations within a single animation file without having to start from scratch with each walk cycle. If you like a particular variation, export the animation layer to its own file. It's a nondestructive way to try out variations; and walk cycles are endlessly varied.
There is no more telling expression of character than the walk cycle. Use a walk cycle to get to know every new character you approach. I create a walk cycle as the first thing I do to test every new rig I'm building, so I can begin to finalize the rig in the context of the character's particular personality.
It is worth noting that the "heavy" and "sad" cycles would be at a slower pace than average, but I present everything at a standard 24-frame cycle for maximum compare/contrast.
A good online resource for further exploring comparative walk cycles is the BMLWalker.