There was no script, just a 10 page treatment. The movie was practically improvised.
The players in the bingo hall scene are not actors and their reactions are real when Brendan Fletcher takes and calls out a number.
Macaulay Culkin was offered the role of Billy, but turned it down.
At least two of the guns in the movie are blank firing guns (Bill Williamson's H&K MP7 and Ruger MP9) loaded with quarter-load blanks, requiring everyone on set to wear hearing protection when in-doors.
The character's name "Evan Drince" is an anagram of "Can Never ID".