When I direct a new animator I haven't worked with before, I will often ask him/her not to forget the most important person in the scene. Naturally, they will agree, but then I ask if they know who that is.
"The main character?" they reply with some timidity.
"No" I will gently say. With a kind sigh and small head shake, I will clue them into one of the most important bits on information for a performer: The most important person in the scene is....
Never forget the audience. They are the one's watching, the reason you are doing this in the first place. They are the ones you are trying to reach, to communicate with, to elicit an emotional response (or a laugh) from. Give the audience the best show possible.
I was recently at a school function where the Principal read a poem on stage, while the school band accompanied between the stanzas behind her. The Principal had obviously practices this many times facing the band, because that's what she did during the entire performance. There was a lecturn there with her poem and a mic. She walked up to it, picked up the paper and microphone, turned and read the poem to the band, leaving us a splendid view of her derrier.
She forgot her audience.
Obviously, this is an extreme example of a no-no. But I try to remember at all times that somebody will be watching. Sounds vain, I know. But if you are going to give a performance, make it a performance worth watching.
It's a delicate balance. You have to become fully immersed in your performance, forgetting the outside world. But at the same time, you must train your subconscious to remember that you are performing. We're not making documentaries here, we are performing.
This is a principle that I don't have to remind live actors of. They instinctively know. But animators work in darkened cubicles, cut off from the outside world. You are separated from your character by the glass monitor screen, you aren't acting in front of a camera and interacting with other actors in real time. So while you sit there, imagine yourself in a theatre full of two hundred other people watching what's on your screen. Are they enjoying the show as much as you are?