Using a simple armature to demonstrate basic principles, stop motion animation master Sáenz Valiente (with Guillermo Galli?) of Argentina explains inertia, timing, acceleration, arcs, and momentum. (Video link) "Movement is not the same as animation," he explains.
The voiceover is in Spanish, but the main points should be intelligible even if you don't speak the language. There's also a version with English subtitles, and don't miss Part 2, where he gets into acting and interpretation.
Also, check out "Frame by Frame: The Art of Stop Motion" from PBS Offbook
One of you might want to try this technique for the Plein Air Persistence video contest.