Contrast in Animation
I think one of the most powerful principles is Contrast. Contrast is obviously not one of the original principles of animation, but it can be found in many of them. Lets explore some of them. Contrast in pose for instance. Having two characters posed differently can tell the viewer a lot about each character and create an interesting dynamic between them. In the example of Bob and Helen arguing, there is a really nice contrast between Bob's hunched over pose and Helen's more upright one. This then gets flipped when Helen becomes more angry. Contrast is also really great in motion. It directs your eye to where you should be looking. An obvious example would be two people talking. You would not want to move the character that is not talking too much so that you could take in what the other character is saying. Another example could be a crowd of people moving around. The person that is not moving is most likely the person your eye will be drawn to. Animation is like a choreographed dance. You really need to understand how to weave the different elements in and out of your scenes. That reminds me of contrast across many scenes. You want to think about it not just on the scene level, but on a more global one.
Hope this helps...