Peepboardpleasure from Davy and Kristin McGuire on Vimeo.
Take a screen. Film a scene(s) where the head is always at the same place in your frame. Project the video onto the screen. Cut a peephole for the head. Then insert a live person behind the screen with his or her head poking out of the hole. In the above video, his and hers are interchanged for some hilarious effects.