Shot in 5 days over the course of 15 months.
Originally titled 'Cillian,' which means 'warfare' in Celtic, the name was changed late into production because cast and crew kept pronouncing it incorrectly.
The mission briefing scene was filmed on two days, months apart from each other, with the four actors sitting at the table on one day and Matt McGorry filmed months later.
Director Tyler Bell already knew all the actors he would cast and only held auditions to determine who would get which parts.