Traditional Principles of Animation/Others
Weight transfer is essential for realistic motion. Watch an elephant or rhinoceros walk through its surroundings. You can actually see (and almost feel) its weight shift from one portion of its body to another. Does a young kitten move the same way? Does a bowling ball bounce like a beach ball? Imagine how unbelievable a feather would appear if, when dropped, it shot straight down to the ground. Instead it sways from one side to another as it descends; its motion provides a clue as to its ratio of mass to surface area, and the resistance of the air.
With the exception of falling, the body is almost always in balance. If you extend your right arm, your left arm, shoulder and torso all pivot and move back. Few people stand perfectly straight. Think of what position other body parts should be in when one body part moves or changes location.
Interesting Note: Walking and running are special cases of falling. When you walk, you constantly cycle through a process of falling forward, restoring balance and falling forward again. Running works the same way except that you spend most of your time falling forward.
No straight lines exist in nature. This statement also applies to natural motion. Watch a person as they swing their arm up in a handshake. Does the arm simply swing straight up? Usually the arm not only swings up, but also swings out from the side and back in again. This subtle motion makes all the difference between an unnatural, robotic move and something that appears life-like.
In contrast, a motion by an intentionally inanimate object should follow Newton's laws, i.e. move in a straight line until it is affected by a force. This provides the necessary contrast that differentiates it from a "sentient" object.