To anyone whose illustration work includes characters, facial expressions are like this computer monitor: if it doesn’t work right, then all the brilliant engineering that went into building the hard drive is wasted.
The human face ranks at the very top of the hierarchy of things the eye is immediately drawn to: if a face is visible in a given composition, the very first thing we look at is its expression. The body expresses action, but the face is a window into someone’s inner life, and the expression of this inner life in a character makes all the difference between a skilled, observant artist (or writer) and a wooden one.
A facial expression that is alive can make up for some weaknesses in proportions (partially because it will keep the eye from wandering away from the face!), but not the reverse – a character with a face like a wax mask is a turn-off.