Here is how to freak out your roommate in a few simple steps.
Requirements: bed, two pillows, dovet and a scary mask.
Step 1: The Body
Place two pillows under your dovet in a line to create the effect of a body.
Step 2: The Head
Place your mask at the appropriate end of the pillows making sure to keep it in the right shape, possibly stuffing it with a woolly hat or something, and then you will have your monster.