According to an on-set exorcist, one of his favorite scenes in the film because of its realism was when the weary psychiatrist expresses, "I didn't know I was in a fight," to which the demon responds, "That's why you're losing."
According to "The Spirit World," a Catholic radio station program interview with a Vatican-approved priest/exorcist named Rev. Gary Thomas (whose experience as an apprentice to a veteran exorcist provided the basis for Le Rite (2011), on which he was also a creative consultant), this was the most realistically accurate film about demonic possession the reverend had ever seen.
In a scene where a priest approaches Nefarious who seems frightened, some of the hair on Sean Patrick Flanery's head is briefly formed into the shape of horns, which then settle back down when he realizes the priest is not taking his role seriously and therefore harmless.
Sean Patrick Flanery has explained that he took this role because it would help him remain true to himself and his faith.