The alley scene after the Christmas party had no directions in the screenplay; it was simply called "Shit Reservoir Dogs".
James McAvoy revealed in an interview that he would drink up to half a bottle of whisky every night so that his character looked rough and hungover.
The film may exist in the same fictional universe as Trainspotting (1996). In the source novel, Bruce recalls crossing paths with Spud and Begbie, among others. Irvine Welsh wrote the source novels of both films.
The closing cartoon credits include a mountain range which has been painted to look like a profile of James McAvoy's face.