In Area B: Room 4, enter the Sphinx puzzle room, then turn around. Click the wall of hieroglyphics to find the red Papers, Please logo on the rock door. Go through the first door on the left, and examine the altar. Collect the small green item behind the vases in the left corner. Return to the door to pass through it. Inside is a movie theater where you can watch a short film featuring Serious Sam after you wait a short time.