Metafictions tell stories in which the physical medium of the story becomes part of the story as, classically, in Tristram Shandy or Don Quixote. In our times, both metafiction and metafilm have proliferated. Examples of metafilm include Buster Keaton's Sherlock Jr., Woody Allen's Purple Rose of Cairo, Alejandro Amenábar's Abre los Ojos, Ingmar Bergman's Persona, the Marx Brothers' Horse Feathers and, in particular, Spike Jonze's Adaptation. In my experience and that of others, metafilmic movies have a peculiarly disconcerting effect, sometimes arousing fear, sometimes seeming comic. Why? Metafilms play tricks on the levels and kinds of our belief (or our suspension of disbelief). To explain the effect, we need to understand how our brains are functioning when we are, as we say, "absorbed" in a film. The answer lies in the fact that reality testing depends on activity in the motor regions of the frontal cortex. But in experiencing the arts, we are not moving or even planning to move. As a result, as Richard Gerrig's experiments show, we momentarily believe (or suspend disbelief in) the film we are perceiving. Metafilm, however, introduces another, more real reality, the physical medium of the film. Metafilm thus sends conflicting messages to the brain about moving. The result is what Freud called "a signal of anxiety." If the metafilmic effect is brief, we laugh. If it persists over time, it can arouse anxiety.