In real life Gerry and Giuseppe did not share a cell in prison.
Gabriel Byrne had purchased the rights to Gerry Conlon's book "Proved Innocent" and had intended to play Gerry himself but decided to let Daniel Day-Lewis play the part and just serve as executive producer instead.
Daniel Day-Lewis kept his Northern Irish accent on and off the set for the entire shooting schedule.
The character of Inspector Dixon is a mix of several police officers connected to the case.
In preparation for his role, Daniel Day-Lewis lost 30 pounds and spent nights in the jail cell on the set as crew members threw water and verbal abuse at him.
Paul O'Grady played a prisoner but was cut from the final print.
Despite his executive producer credit, Gabriel Byrne distanced himself from the project, chiefly due to the "artistic" liberties that were taken with the details of the story.