Aidan Quinn's character, Gary Hallet, has something called heterochromia iridium. Heterochromia (from the Latin "hetero" for different + "chromia" for color) iridium is a harmless condition in which one iris is colored partially or completely different than the other iris. Although it can be rare in humans, there are some celebrities with this condition, such as Kate Bosworth, Mila Kunis, and Jane Seymour to name a few.
The character Jimmy Angelov was originally named Jimmy Hawkins, and was a Texas redneck. The part was rewritten as Eastern European for Goran Visnjic after director Griffin Dunne saw him in Welcome to Sarajevo (1997) and Madonna's "Power of Goodbye" video.
The black dog that follows Sally's (Sandra Bullock's character) husband Michael (Mark Feuerstein's character) on the day of his death is no accident. According to several cultural traditions (mainly those of the British isles and Latin America), a sign of impending death can take the form of a black dog.
When Sally and Gary are talking in his motel room he is wearing a plaid shirt. After she leaves and we see him looking out the window he is wearing a white shirt.
In the scene when the women are in the circle, near the end of the film, Sally notices that the spell is hurting Gillian and tells the others to stop it. She does this while standing still in her place, in the next shot she has ran half way round the circle and returns to her place even though we never see her move. The background music also jumps at this point as well.