Holds and keys are the drawings you see on a conscious level in animation. That doesn't mean that the drawings you don't see have to be straight inbetweens of the keys.
You can get a lot more meaning, feeling and power by controlling the drawings that are on the way to the moments you absorb consciously.
I found this out by studying Rod Scribner's animation and live action frame by frame.
This part of a comercial for Barq's Root Beer was only 5 seconds long and I wanted to get a lot of stuff crammed into it. That meant no pauses or hookups, less antics or at least shorter ones, so this became an experiment in making things read fast. No drawings could be wasted.
I drew a bunch of keys and had Elinor Blake (April March) animate the commercial with these theories in mind.
Here's one of her early bands, The Shitbirds.
I wonder why so many animators are also musically talented.HUMAN EXPRESSIONS
Human expressions are generally a lot more specific and interesting than animated expressions.
Watch Norton's face crawl around and do all kinds of extra things that the script doesn't require, but add extra life to the story.
This is very hard to do, if not impossible in animation. A lot of these expressions can't be translated well in line; they need the subtle shadows and shapes made by the many muscles in his face.
If we want to compete with this specificity, we have to be more exaggerated in our accents, but we can do very subtle expressions in between the accents as you can see in the Barq's frames above.
You can't compete at all with this kind of acting if you are habituated to approved animation stock expressions - no matter how smooth the action is. 
Plus it's a lot more fun to customize specific expressions to your characters and story. Of course for some mysterious reason most studios won't allow it. They all say they wanna compete with live action, but no way in Hell will they actually let you do what's required.
You can really see how much subtlety is in Art Carney's acting when you watch the clip of one simple line of dialogue. A Hell of a lot of stuff happening for just one script sentence!http://www.cartoonthrills.org/blog/01Principles/DialogueActing/NotThatImSelfish.mov
More from Barq's and this subject in another post...
