Look who's (not) talking.

Here's a trick. Watch a movie or TV show or whatever. Look for a two-shot or group shot.
Somebody will usually be talking. The fun part is... look at who isn't talking. (mind blowing, isn't it?).

In this scene from Back to the Future, it's Doc Brown's line. But Michael J. Fox doesn't stop acting because he's not speaking, he's still present in the scene: reacting to what the other players say and to what's going on.

I find this a pretty good litmus test for character animation.

Can you still see what's going on in the character's head when another player is speaking? What would you think is Santa Claus's inner monologue right here?

How About Daffy, or Elmer Fudd?

I find looking at the nonspeaking characters, and gauging their reactions and acting in those moments, in a two-shot or group shot is a good way to determine between what's good animation...

...and what I would consider otherwise.