Charles Bronson was not allowed to see the film, but said that if his mother liked it, that would be enough for him. According to Refn, his mother loved it. In 2011 Bronson was finally allowed to see the film and called it "theatrical, creative and brilliant".
Charles Bronson shaved off his moustache and sent it to Tom Hardy so that it could be made into a loose-moustache for Hardy to wear.
Nicolas Winding Refn was not allowed to meet Charles Bronson in person since he is not from Britain, but was allowed to have two phone calls with him. Tom Hardy met with Bronson several times and the two became good friends. Bronson was impressed with how Hardy managed to get just as muscular as he was and how well he could mimic his own personality and voice. Bronson has stated he believes Hardy was the only person who could play him.
The line "it was absolute madness at its very best" was written by Charles Bronson himself for the film and told to Nicolas Winding Refn during one of their phone calls.
When the real life Charles Bronson was moved to Parkhurst Prison in 1976, he befriended the notorious Kray Twins. Coincidentally enough, Tom Hardy would later portray both of the Kray twins in the film Legend (2015).