I will start by saying this; The 12 Animation principles are the cornerstone of animation, they are taught to every learning animator as the guidelines to the "Illusion of Life" as the cover suggests.
However, I will start with my days as a student animator struggling with "the checklist" and it seemed like most people just could either have the innate ability to pick up the idea of movement of life or got stuck on the mechanics of it all.
The 12 Animation principles are only written by two of the nine old men and it only covers the basics to start off the training that Disney started to provide its animators, it is like getting a photocopied hand out of how to start your job on the first day, without no back up or guidance of a experienced animator. This means that you don't learn about beats or rhythm, plus the jerkiness (I leave you to find a better word, maybe clumsy?) of the human condition and behaviour.
Shoved through the idea of timing and spacing in an instant with you being only shown a visual representation of both. This is not what you see when you go and see an animated film and if you are lucky to see some pencil test you will notice the wavy line next to most of the keys symbolising the timing and spacing of the scene (this is got a little weirder in 3D animation software as you have both the dopesheet and the timeline,more complicated than that the idea of offsetting keys by moving them down the timeline means that you can start getting miscellaneous keys everywhere).
Not that I am saying that the principles are anyway shape or form wrong but they are shown in paramount, it would of been nice if the other Nine Old Men had actually kept quite alot of their notes, most are just two or three pages long and some have been photocopied nearly to extinction, some being lost forever.
There is although, that all the Nine Old Men had different workflows and quite possibly different principles as it would be only true that they had studied in great detail how to instil life into the paper with a pencil.
Some of them hummed or started using their pencils or hands to drum out the rhythms on their desk, hopped up and acted it out (not that this is what video reference is for but maybe quite a lot of you will have the same problem as I did, you act different if you know that you going to record yourself this leads to tenseness, no conviction, gestures seem very artificial, acting to camera or from another angle doesn't give you right poses and the timing seems shallow and beats seems not there).There is a line, I forgotten from who but it goes like this "to work as an Animator at Disney you have to know how to build a house, to sing, how to dance the waltz and act " you can add many other things behind the last one but it sums up what quintessentially an animator is.
We are creators, we don't bring the illusion but life itself for the short amount of time that your work is interacting it is "alive" it should not be not be termed anything else and it should not be considered to be inferior, it is living and breathing. This is why we love being animators.
Thankyou for reading and keep animating.