In the movie, Farrell and Gleeson play Irish hit-men hiding out in Bruges, Belgium, after a job goes horribly awry.
"When I have to do a dialogue or dialect, there's a danger," Farrell told reporters this week.
"It can be one of two things, just generally," he explained. "It can be an avenue into a character and a form of kind of separating yourself from your own natural patterns and sound, but there is also a danger -- and maybe it's before I hadn't worked as hard as I should -- where there have been times when I heard what I was saying.... When you're in the middle of a scene, that's the worst thing in the world, to be aware of what you are saying and to be taken out of any given moment. So, it was incredibly liberating to just use my own vernacular."
Farrell has also starred in the films "Miami Vice," "Alexander," "Daredevil" and "Phone Booth."