An excerpt from Gene Deitch's book, How To Succeed In Animation (Don't Let A Little Thing Like Failure Stop You!).

Technically, the production method of film animation remained basically unchanged since Raoul Barre invented the peg hole, and pegs to fit into them, and John Bray invented cels. That was in 1914, ten years before I was born. More or less the same methods were used for nearly 80 years, until the advent of computer animation, scanning, coloring and compositing. We were still producing our little animation movies on Bray's cellulose-acetate sheets until just a few years ago — and we're still using Raoul Barre's peg holes today when we're doing drawn animation.

The animation principles we work with today were developed and perfected in one incredibly explosive decade. The great advances in animation were made and codified within the 10 years between 1930 and 1940 at the Walt Disney Studios on Hyperion Boulevard in Hollywood. Whatever we may think of the artistic taste of Disney, we cannot discount the incredible advances of animation technique that evolved during this golden decade in his studio, hardly surpassed to this day. That single decade brought animation from the crude Steamboat Willie to the highly polished Fantasia! How much better is animation today?

Every animated film made today uses those same basic principles developed at the Walt Disney studios during the 1930s. They still apply, no matter which technology is used. They may be difficult for you to understand completely, without fuller explanation or demonstration, but perhaps they will give you something to think about. They were all printed in Frank Thomas' and Ollie Johnston's landmark book, The Illusion of Life: Disney Animation.

The 12 Principles of Character Animation, as developed at the Disney studio:
1. Squash and Stretch. (Shape distortion to accentuate movement.)
2. Anticipation. (A reverse movement to accent a forward movement.)
3. Staging. (The camera viewpoint to best show the action.)
4. Straight-ahead vs. Pose-to-pose action. (Two basic procedures.)
5. Follow-through and Overlapping action. (Nothing stops abruptly!)
6. Slow-in and Slow-out. (Smoothing starts and stops by spacing.)
7. Arcs. (Planning the path of actions.)
8. Secondary Actions. (A head might wag while the legs walk!)
9. Timing. (Time relations within actions for the illusion of life!)
10. Exaggeration. (Caricature of actions and timing.)
11. Solid drawing. (Learn good drawing to be a good animator!)
12. Appeal. (If our characters are not appealing, then all is lost!) If I may quibble, I would add:
13. Mass and Weight. (And preserve volume!)
14. Character acting. (Thinking of the character as a real actor.)