Eye dart is the fast moving micro-changes of eye direction. Characters eyes are the element where the attention is most directed to. Eyes are very expressive and are key for communicating thoughts and feeling.
changes of eye direction are usually accompanied by blinking.

animate the brows: eyes and eyebrows should be animated together such as it happens on a blink when the eyes and eyebrows tend to close and open together (which is something that was useful in my performance animation: after I animated the brows I animated the eyes up and down according to their movement)

eye direction can help maintain the connection between characters or a character with an object: making sure a character is looking at the object his or her speech is aimed at may help to improve the overall intention of an action.

Eye darts:

eyes often dart from side to side moving around rapidly as they do if one think or talking. Eyes dart in animation happen fast so maximum three frames and they are used so to not make eyes look dead and keep them alive, however they should not move too much otherwise they would appear detached from the body action.

”eyes are the window to the soul”

in some animation the eyes have to carry all the expression as it happens in the phone character in toy story 3 where the eyes and eyebrows are the only mean to unrestricted the characters expressions and they are enough.

eye lids can carry a great amount of expression in the eyes: wide eyes can give a blank look. In the film “Robots” they had a problem on how to design the eyes of the robots characters. So to create an appealing eye expression they decided to use the down eyelids.

when changing the eye direction is recommended to use an eye link as a transition otherwise it would result in a odd animation because we always blink in real life.

there are different types of blink:

a light blink, a heavy blink or a half blink. And when animating the eyes lids the eyebrows should move as well in unison. Slow blinking can show the character thinking compared to a fast blinking. The eyelid carries the moment of the pupil usually. An offset eye blink for the eyes can add texture to the animation.

Williams, A., 2021. How to Animate Eye Darts. [online] Animationapprentice.blogspot.com. Available at: <http://animationapprentice.blogspot.com/2019/07/how-to-animate-eye-darts.html> [Accessed 13 December 2021].