Comic Relief

The Comic Relief is the inclusion of a humorous character, scene or witty dialogue in an otherwise serious work, often to relieve tension. 
Comic relief usually means a releasing of emotional or other tension resulting from a comic episode interposed in the midst of serious or tragic elements in a drama.

Normally, when I laugh while I'm watching an horror film is because I've just scream or something like that. Also I laugh with some faces people do. 
One film I remember I laughed but at the same time I was super scared was with The Exorcist, when the girl is in the bed screaming, vomiting in the face of a man, moving her head...