In order to ensure that Johnny Depp's performance was true to the actual James 'Whitey' Bulger, Scott Cooper hired some of Bulger's old associates as consultants to the film with each of them praising Depp's performance by simply saying, "That's Whitey."
In an interview on Jimmy Kimmel Live! (2003), Johnny Depp admitted that he broke his rule of not watching his own movies with this one. He was at the film's debut in Venice. He was about to skip out of watching the film, but the director Scott Cooper asked him to stay, so he did.
Upon seeing Black Mass, the real Kevin Weeks took issue with the film as a whole, calling this a "work of fiction". He later ripped into Johnny Depp's portrayal of James 'Whitey' Bulger as well as the portrayal of Stephen Flemmi by Rory Cochrane and the portrayal of himself by Jesse Plemons, stating that the performances were "way off". He also took issue with the film showing his character involved in crimes he vehemently denies being involved in.
Jesse Plemons gained weight for his role as Kevin Weeks and was subsequently asked by the producers of the television series Fargo (2014), his next project, to keep the weight on, as he portrays a character who is a butcher by profession.
Revere Beach in Revere, Massachusetts, just north of Boston, was transformed into Miami Beach for the Miami scenes. Filmmakers added real palm trees to the beach and dressed up a local pizzeria to look like a Cuban café.
William Bulger, president of the Massachusetts State Senate, is so short that one of his critics labeled him a "corrupt midget" during a rant. Benedict Cumberbatch is eight inches taller than the real-life William Bulger.
The title of this movie, "Black Mass," has several meanings. The main meaning of the term "black mass" is a ritual in witchcraft or satanism that is an inversion of the Catholic Church's traditional Mass for the purposes of evil instead of good. One of the things that nearly all of the criminals depicted in this movie (including the Irish mobsters, the Italian mafiosi with whom they were feuding, and FBI agent John Connolly, who was abetting Bulger) had in common was that they were Catholics, even though their murderous activities were in direct opposition to the values taught by the Catholic Church. The title is also a play on the color-themed nickname of its main character (Whitey Bulger), as well as a nod to the corruption endemic to the city and state in which the story is set (Mass. is an abbreviation for Massachusetts).
Cult icon Mark Mahoney was cast as Mickey Maloney without reading or auditioning for the role on the strength of his performance in Blood Ties (2013). The famous Venice Beach tattooer, scenester and owner of the Shamrock Social Club was "audacious, to say the least" (according to executive producer James Packer) in regards to the remuneration he demanded for his relatively minor role. Director Scott Cooper insisted that an agreement was to be reached with Mahoney - partly because he was frustrated by constant difficulties securing, then losing, then (for the most part) re-securing key cast members, and partly because "the amount of money he demanded and the manner in which he demanded it was in someways the best audition I've ever seen!.. it reminded me why I wanted him for the part in the first place!" Cooper allegedly quipped on set. In his downtime during shooting, Mahoney would set up and put a shamrock tattoo on any member of the cast and crew who wanted one.
In an early scene, Benedict Cumberbatch references an incident that happens on Baker Street. Cumberbatch is perhaps best known for playing Sherlock Holmes, who lives at 221B Baker Street, on the television series Sherlock (2010).
Corey Stoll plays a district attorney who is hunting for agents who have made a questionable deal with an FBI informant. In the fourth season of Homeland (2011), Stoll himself played a CIA operative who made illegal deals with local informants.
The trivia items below may give away important plot points.
When he was arrested in 2011, James 'Whitey' Bulger had been on the FBI Ten Most Wanted Fugitives list for 12 years. The $2 million reward the FBI offered for information leading to his capture was larger than that for any other fugitive on the list except Osama bin Laden.
The ending of the film briefly shows the capture of James 'Whitey' Bulger, and it's factually correct. Living under the alias Charlie Gasko, Bulger was arrested in the parking garage of the Princess Eugenia Apartments in Santa Monica, California on June 22, 2011. According to FBI agent Scott Gareola, Bulger immediately put his hands in the air. When he was ordered to get on his knees, Bulger replied, "I ain't getting down on my fucking knees." He was wearing white pants, and reportedly did not want to dirty them on the oil-stained garage floor.
Various Boston-based gangsters were omitted from the movie. For instance, there is no mention of Howie Winter, James 'Whitey' Bulger's boss, who was arrested in 1979. Another key, criminal associate left out was Kevin O'Neil, the owner of Triple O's Lounge who laundered money for the gang. Perhaps the biggest omission was Patrick Nee, an arms trafficker, who got Bulger involved with the IRA and was also involved with the murders of John McIntyre, Brian Halloran, and Michael Donahue.
Joel Edgerton chose not to meet the character he played, John Connolly, because he was aware that Connolly's version of events differed drastically from the way they were portrayed in the film. "I felt like even if there was a possibility of me going to see John, something would have felt slightly unfair - the idea of me strutting into prison to learn from John, knowing that he has one version of events and the film has a different version."