The head is made up of 2 major parts:
1) the Cranium - which is generally solid (except in exaggerated instances of cartoon license)
2) the Jaw Area- is more movable and can squash and stretch
The upper and lower parts of the head are connected by MEAT:
cheeks, and upper lip.
The rest of the features have to conform to what positions the cranium, jaw and cheeks are in.
Eyes sit in the cranium but are affected by the mouth and cheeks.
Eyebrows both affect and are affected by the eyeballs.
The mouth is affected by 2 things:
1) Whether the jaw is open or closed.
2) The shape of the lips.
Every part of the anatomy affects the others.
If you change the shape of one thing, the other parts have to change (to a lesser extent with it).
The farther away from whichever muscle is moving, the less affected the other parts are.