The Kovak & Son Funeral Home is supposed to be set in Dixon, Illinois, as seen in the letter from Italy, that Michael gets at the end of the movie. However, the area code for the phone number on the funeral home sign is "312", which is an area code for Chicago (over one hundred miles away from Dixon).
The trivia items below may give away important plot points.
The writing on Father Lucas' (Sir Anthony Hopkins') wall ("Fi sydd bia fe nawr") is Welsh (just like Hopkins), and translates to "He belongs to me now". Which is what Lucas utters when he is possessed.
The image used to describe the demon Baal is usually that of a man, a cat, a toad, or a combination of all of them. Father Lucas' (Sir Anthony Hopkins') house is infested with cats, as well as toads in the fountain.