The 1935 Rolls Royce Phantom that was used in the film is a one of a kind and actually belongs to Nicolas Cage. The filmmakers were trying to find a really cool classic car and Cage offered it as an option to use.
The Chinatown sequence took 5 months to plan. Half a ton of confetti was used in this sequence.
The idea for the film was largely Nicolas Cage's as he wanted to explore a mystic world with a magical character.
One of the people that Horvath summons is Abigail Williams. At age 12, Williams was the first person to make the accusation of witchcraft in 1692 Salem, Massachusetts. This led to the Salem witch trials and the deaths of at least 20 people. There is no definite evidence of what happened to Williams after the trials ended.
The Incantus - the sorcerer's Bible - took 3 months to be put together and was deliberately aged to resemble a tome from centuries back.