It’s my casual observation that the bathroom scene is a bit of an institution in horror films, probably ever since poor Janet Leigh famously bought it in Psycho. Off the top of my head: The Collector (1965); classic French chiller Les Diaboliques (1955, predating Psycho); Carrie (freaking out at menstruation and what she sees as a literal ‘curse’); What Lies Beneath (with Michelle Pfeiffer); 90s slasher flick Copycat (public toilets); Final Destination (enforced ‘suicide’); John Carpenter’s latest, The Ward (very scary) and in my most recent reviewing pile—The Roommate and No Time To Fear. I’m sure there are more.
Stephen King wrote a short story in which a man is terrorised by a tentacular finger emerging from the plug hole in his bathroom hand basin, which retreats if anyone else enters.
We’re vulnerable in the bathroom. Hot water gushes, the room steams up, and suddenly there’s something nasty in there with us. There are plenty of reflective surfaces for apparitions to appear in. And more often than not, we’re alone.