Joel Edgerton was cast as John Connelly but after Johnny Depp dropped out, the project was shelved. After Depp agreed to return, Edgerton was unavailable and Tom Hardy was cast. Turns out, Hardy was just a rumor and in the end, Edgerton returned in his role.
Upon his arrest in 2011, James 'Whitey' Bulger had been on the FBI Ten Most Wanted Fugitives list for 12 years. The FBI offered a larger reward for information leading to his capture (US$2 million) than for any other fugitive on the list except for Osama bin Laden.
Leonardo DiCaprio's influence may have contributed to director Scott Coopers interest in taking on this project, since DiCaprio had helped produce the Cooper's previous movie Out of the Furnace (2013), and had starred in The Departed (2006), which is based on a Whitey Bulger-like character in Boston.