The Telegraph looks at Why We Scream:
We all have a core set of five facial muscles that control our ability to produce standard expressions which convey anger, happiness, surprise, fear, sadness and disgust. But there are up to 19 muscles present in the face, and many people do not possess all of them.
The risorius muscle, which controls expressions of extreme fear, is found in only two thirds of people.
via Mind Hacks