The original script envisioned John Milton as a 70-year-old man and the producers were looking for an actor reasonably close to that age to play the part. When Nicolas Cage expressed interest, however, they decided to make the character's age irrelevant and cast Cage instead.
At some point, Milton mocks the Accountant by asking if he thinks he is Anubis, to which the Accountant just shrugs. In Egyptian mythology, the jackal-headed god Anubis was the one in charge of deciding whether a soul belonged to Heaven or Hell by weighing his heart on a scale.
Nicolas Cage originally wanted to shave his head and sport a full tattoo on his cranium. He was talked out of this.
Nicolas Cage said that one of the reasons he took on the part was the sequence where Jonah King shoots out Milton's left eye. There had been a similar sequence for Cage's character in the film Season of the Witch (2011) but it was cut so that film would keep its desired PG-13. Cage loved the notion of it being part of a crazy film that was aiming for a hard R rating.
The coin the Accountant uses throughout the movie is an Obolos, from ancient Greece. In those days, Obolos were placed in a dead man's hand (or two over his mouth so the first time he opens his mouth the payment falls out into Charons hand) by mourners as payment to Charon the Ferryman, who was in charge of crossing souls over river Styx into Hades, the Greek version of Hell. This would support the Accountant's role as a retriever of lost souls.
Todd Farmer: Wrote the screenplay for the film, as boyfriend of Amber Heard's character. He is discovered to be cheating on her, gets badly beaten by her, and is later tortured for information by The Accountant.