People involuntarily express horror when they feel fear.
The facial expression of horror is three circles - two wide open eyes and one wide open mouth.
In addition to the horror expression, fear also causes involuntary reactions such as elevated heart rate and sweaty palms.
People also scream involuntarily when they feel fear.
However, people only scream involuntarily when they fear for their life. This is what people mean by a blood-curdling scream.
People can express horror voluntarily.
People can make the three circles face without feeling fear.
Seeing the horror expression makes others feel fear.
Seeing somebody expressing horror makes you feel fear - just like the sight of spider or snakes. A popular prank involves asking others to look at a computer screen that suddenly changes to a face showing the horror expression. The viewer always jumps away in fear.
Similarly, hearing a blood-curdling scream also makes you feel fear.
For more about emotions, visit: Happiness Dissected