Hi:
I highly recommend to all animation students the book:
animator's survival kit by Richard Williams.
I love this book, and enjoy reading very much.
In the book, the author says that the most important
principle of animation is 'anticipation, action, and follow thru'.
Which only means 'before action, action, and after action'.
The author also adds that all the other animation principles
rests on the beforesaid principle.