Mirrors in horror movies are also used to say something about how we face fear.
In films such as Candyman (1992) and Paranormal Activity 3 (2011), characters face the mirror, and quote a name (like “Candyman” or “Bloody Mary”) three times in a row to summon a force of darkness. In these situations, people are facing themselves and their own fears, and looking for some sign that our “belief” can re-shape reality itself. We look in the mirror at our own expressions, our terror growing...
On a more frequent basis, mirrors are utilized in vampire films such as Fright Night (1985), because vampires don’t cast a reflection. The mirror shows only empty space where Jerry Dandridge (Chris Sarandon) or other “monsters” should be. In other words, a mirror reflects life, but not the undead.
The mirror has proven a potent symbolic and visual device in non-supernatural horror films too. In Brian De Palma’s Dressed to Kill (1980), for instance, Dr. Elliott (Michael Caine) hides a terrible secret about himself, a fracturing of his very psyche. Virtually each time his alternate identity, Bobbi, is mentioned even in passing, De Palma cuts to a view of Elliott looking in the mirror, an important visual cue of the psychiatrist’s schizophrenic nature.
On a very basic level, the mirror is also a useful device for generating jolts or jump scares. A character runs into a room, slams the door...and something evil is reflected on the mirror hanging there. We've seen this kind of surprise jolt in films such as Phantasm (1979) and even, to a degree, in Halloween IV: The Return of Michael Myers (1988), wherein we witness The Shape illuminated by lightning....in a mirror.
One of my all-time favorite mirror shots appears in this film as well. The Shape lifts his knife, mirror behind him on both sides, and we see an endless lines of stalking Boogeymen in the frame. This was a perfect visual representation of the film, which featured on more than one occasion "false" or "multiple" Michaels to confuse the authorities (and audiences).
The mirror is featured in (but not limited to…) such horror as: Phantasm (1979), The Boogeyman (1980), Dressed to Kill (1980), The Sender (1982), The Entity (1983), One Dark Night (1983), The Initiation (1984), Fright Night (1985), Cassandra (1986), Evil Dead 2 (1987), Hello Mary Lou: Prom Night II (1987), A Nightmare on Elm Street III: Dream Warriors (1987), John Carpenter’s Prince of Darkness (1987), Halloween IV (1988), Night of the Demons (1988), A Nightmare on Elm Street IV: The Dream Master (1988), Poltergeist III (1988), The Church (1989), Single White Female (1992), Candyman (1992), Paranormal Activity 3 (2011), Dark Shadows (2012).