The trivia items below may give away important plot points.
The writing on Father Lucas Trevant's (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 Trevant's (Sir Anthony Hopkins') house is infested with cats, as well as toads in the fountain.
When Father Lucas Trevant (Sir Anthony Hopkins) hands Michael Kovak (Colin O'Donaghue) the umbrella and says, "it's going to rain", on a clear day, foreshadows the fact that Father Lucas is possessed. Earlier, he had told Michael that the easiest way to tell if someone is possessed is if they can know the unknown.