The boy who plays Daniel Radcliffe's son is his real godson, casting suggested by Radcliffe himself, which helped him establish an authentic relationship between father and son.
The music boxes and mechanical toys in the nursery scenes were not created for the movie, but were genuine antique toys from the period, loaned to the production by a collector.
The biggest grossing British horror film in 20 years.
The location used for the fictional Nine Lives Causeway (leading to Eel Marsh Island) is Osea Island's tidal causeway situated at the estuary of Blackwater River in Essex, England, United Kingdom. Due to tidal conditions, the cast and crew were restricted to only 4 hours of working time per day at that location.
(at around 20 mins) Arthur opens the curtains and the light reveals three statuette monkeys. These are the Japanese "three wise monkeys," Mizaru, Kikazaru, and Iwazaru, who "see no evil, hear no evil, and speak no evil." Although their origins (and the phrase accompanying them) are disputed, it is generally believed that they are part of a Japanese (particularly Buddhist) "Golden Rule" to avoid spreading evil.