When asked what they did to prepare for the film, Brendan Gleeson said he went to find a coat and Cillian Murphy said he grew a beard.
Although set in and around Dublin, most of the film was shot in London because it was cheaper.
Brendan Gleeson said the name of his character Perrier was a joking reference to his sparkling personality.
Michael McElhatton, Conleth Hill, Liam Cunningham and Francis Magee have all appeared in Game of Thrones (2011) as Roose Bolton, Lord Varys, Davos Seaworth and Yoren respectively with Jim Broadbent to appear in season 7.