At animation, you should consider many things, and while keeping in mind the basic principles is great, sometimes it just wouldn't always suffice.
It is a well known fact that an animator is really an actor (I would like to consider my self as a shy one).
An actor, just like a magician who needs to perform in front of an audience, should maintain the illusion he wishes to portray, and for doing that, sometimes includes to control the audience point of view and visual focus.
He needs to manipulate the audience eyes to look to one place while he's making an "magic trick" he doesn't wish the audience would see in a different place (sometimes it even could be used to conceal a bad animation).
There are many ways to do so, and I'm not going to get much deeper into it, but an animator could use his animation of course to guide the audience eyes by using his animation (such as body language), by a correct staging and composition, by using his props and surroundings in the scene, etc.
I would like to thank Shuki Gamliel which is a great animator and a good friend, who gave us a short lecture about this subject (it was mainly on using a technique called NLP but I felt it also fit well in animation and I've waited too long to write this post).
Here you could find two examples of how easily you can manipulate the human eye.