Walt Disney once employed a team informally known as the "Nine Old Men," nine of the greatest animators of all time and the artistic forces behind nearly the first forty years of Disney animation. If you watch classic Disney movies today, their bold and fluid animations are still astounding and a great artistic accomplishment. Two of Nine Old Men, Ollie Johnston and Frank Thomas, explained the "12 basic principles" behind their iconic style in The Illusion of Life, a book that is now considered one of the most important animation reference works.
But let's face it, this is about animation. You want to see it in motion, right?
Artist Cento Lodigiani put together a great 3-minute clip demonstrating each of the twelve principles, from basics like "staging" to advanced ideas like "follow through." Lodigiani is not a Disney animator, but it's easy to see how many of the illustrated principles apply to films like Snow White and Fantasia. Especially "secondary action," which you can see at play in "Whistle While You Work."
There's of course more than twelve principles to quality animation, but this is a fun, non-technical peek into what goes through the minds of great animators.