Sometimes a scary scene can go on for what seems like forever - in cases like this you'll need to make use of The Tilted Hat method. Simply adjust your ball cap so the visor is blocking out most of the screen, then adjust it back once the scare-inducing set piece is over.
The advantage to using this method is that you can keep your eyes open the whole time and still be able to catch the gist of what is happening on screen.
This way, when your friend leans over saying, "Oh man that demon totally ripped that guys face off!” you can respond quickly with "So awesome!" They'll have no idea you didn't watch a second of the frightening footage.