The masked killer is a convention used in a number of the horror movies we have watched, most notably in Halloween and Scream. In Halloween, Michael Myers wears an expressionless rubber mask which acts both to create a fear of the unknown and to detract from his humanity. By hiding the killer's identity from his victim, this convention creates a deeper sense of fear in both the victim and the viewer. The fear of the unknown is something which is explored in a lot of horror movies, and the air of mystery around the killer's identity as a result of him being masked intensifies this fear.
In Scream, the mask itself is the thing which creates fear. By using a grim reaper mask, Billy and Stu are creating fear in their victims in two ways: one, by masking their identity from the people they are killing and the town in general and two, by using a mask with a frightening expression on its face, adding extra horror to the deaths of their victims. Scream also uses this convention to added effect by having two killers behind the mask, so the viewer, in retrospect, is never sure whether it is Billy or Stu committing the murder.
The effect of the masked killer in both movies is to create an added air of suspense and fear. The lack of identity and the ambiguous nature of the killer creates distrust in the viewer of all characters. This helps to drive the plot and keep viewers engaged as it allows the question to remain: which character is the killer?
Since both of these films fulfill the conventions of a "slasher", the use of a masked killer to create mystery plays an important role in setting the overall tone of the film. Not knowing who is committing the murders acts to set the viewer on edge, which spills over into other aspects of the movie and makes the viewer suspect everyone. This is important because it allows the director to place clues as to the killer's identity, which in turn can provide a 'twist' ending when the killer is somebody no one expected.
In Halloween, although we as viewers know who the killer is from the opening scene, the characters do not. This adds to their fear, making the film more engaging for the viewer. This is a slightly less traditional form of the convention, as there is less mystery and fear for the viewer. In Scream, a more conventional approach is taken. The killer's identity is hidden from the audience, and a number of clues, red herrings and false trails are laid out to make sure the mystery remains. This keeps the audience guessing until the final moments of the movie, and even then there is still slight ambiguity as to whether or not the killer is really gone.
The use of this convention in both films added to their commercial success as it was both something that the viewers would expect, and something that could still shock and thrill them. This fits into the general concept of "safe" fear- the viewer can be frightened whilst they still have the knowledge that they are not in any danger, and still within their relative comfort zone.
