Tom Hardy had 35 shooting days in which he played both brothers on the same day. He would film the scenes with the brother who had the most dialogue first, then go back into hair and make-up to be transformed into the other brother.
Tom Hardy played Britain's most notorious prisoner, Charles Bronson (a.k.a. Michael Peterson), in Bronson (2008). Reginald Kray and Bronson knew each other while serving in prison together. After their first meeting, Kray described it as "The most frightening prison experience I've ever had."
In real life, Ronnie Kray kept his homosexuality somewhat private. He saw it as a weakness that could be exploited. It was well-known within his circle of acquaintances, and those who knew him shared that he even came out to his two brothers and mother. He was often seen with attractive young men, which everyone understood to be his lovers, but it was not openly discussed the way it is in the film.
Brian Helgeland first learned of the Kray twins when he was working on an aborted biopic about Led Zeppelin. He had been accompanying Jimmy Page and Robert Plant on a world tour when he noticed one of their entourage was missing a finger. When he asked how he lost his finger, he was told that the Krays had cut it off. Intrigued, Helgeland set out to find out more about the notorious gangsters.
The film became the highest-grossing 18-rated British film of all time in the U.K., surpassing Trainspotting (1996).