Johnny Depp and Black Mass director Scott Cooper on getting under the skin of Boston gangster Whitey Bulger

Black Mass is based on a biopic of the organised-crime boss and convicted murderer, who, say actor Johnny Depp and director Scott Copper is a “very complicated” character.

Johnny Depp is barely recognisable as Whitey Bulger in the Boston-set film Black Mass. Warner Bros. Entertainment via AP
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There are gangsters ... and then there is James “Whitey” Bulger.

Once on the FBI’s 10 Most- Wanted Fugitives list, the Boston native is serving two consecutive life terms plus five years (which sounds a little redundant) in a Florida penitentiary, after he was convicted of 31 crimes, including involvement in 11 murders.

Yet, that is only one part of the story of a criminal who was also an informant for the FBI, and whose brother, Billy, climbed to the upper echelons of Massachusetts politics.

This murky mingling of political, federal and gangster power fascinated actor-turned-director Scott Cooper, whose film Black Mass dramatises Bulger's epic reign of terror.

“In South Boston in the 1970s and 1980s, certain lawmen and certain criminals were virtually indistinguishable,” he says. “You have an intersection of politics and crime that still continues today – it’s something that’s fascinating and makes for compelling cinema.”

As Whitey Bulger, Johnny Depp delivers one of the most chilling performances of his career – making his turn as 1920s gangster John Dillinger in Michael Mann's 2009 film Public Enemies look tame in comparison.

“One side of him was this ­businessman who, within the language of that business, did what he had to do,” says Depp, referring to Bulger’s penchant for extreme violence. “And there was another side of him, the loving family man ... very dedicated to his mother and his brother.”

Describing Bulger as “very complicated”, Depp is well aware he has to be careful – at the film’s premiere, at the Toronto International Film Festival, he angered some relatives of Bulger’s victims when he said there was “a kind heart” in the killer.

Bill St Croix, the brother of Deborah Hussey, who was strangled by Bulger in 1985, said it was “a very stupid, insensitive comment. There’s nothing humane about Jimmy Bulger.”

Still, Bulger was undeniably well liked by some. When Cooper met a woman in Boston that he wanted to cast as Bulger’s mother, she refused.

“She said: ‘Scott, I don’t want to be in the film. Jimmy and Billy Bulger were very good to me, very good to this neighbourhood, and I have no interest, thank you,’” the director says.

Depp adds: “He would take little old ladies’ groceries into the house and then 10 minutes later might be bashing someone’s skull in ... but to him, that was what he knew. That was all he knew.”

While Depp has always been keen on transforming himself on film, Bulger is one of his most arresting creations yet. With receding hair slicked back, his dark eyes are covered with bright blue contact lenses.

“They were hand-painted,” says Depp, “They needed to be piercing. They needed to cut right through you.”

When Bulger’s lawyer, J W Carney Jr, visited the set, he was taken aback.

“I’ve known this man forever,” he told Cooper. “And to see Johnny Depp portray him the way he does, it’s uncanny and chilling.”

Although Depp contacted Bulger, through Carney Jr, the criminal “respectfully” declined to meet the actor.

"I don't believe he was a great fan of the book," says Depp, referring to the film's source text, Black Mass: The True Story of an Unholy Alliance Between the FBI and the Irish Mob, by Boston crime reporters Dick Lehr and Gerard O'Neill. "I also don't believe he was a fan of any of the books written about him."

Maybe not, but you suspect he might secretly enjoy a film with a cast that also includes Benedict Cumberbatch as Bulger's brother, Billy, Fifty Shades of Grey star Dakota Johnson as his beleaguered wife, and Joel Edgerton as FBI agent John Connolly, a childhood friend of the Bulger brothers, who ultimately recruits Whitey as an informant. And that's before even considering an ensemble that features Rory Cochrane, Jesse Plemons, Kevin Bacon and Adam Scott.

A perfect storm of acting talent? No question. And you can expect Black Mass to be a leading contender during this year's awards season, just as Goodfellas and The Godfather once were.

However difficult the subject matter or controversial the characterisation, given that this is a brutal story with real victims, the film offers audiences the vicarious thrill of living a life on the lam.

As Edgerton puts it: “It’s a great way of standing on a little stool and looking through a window at bad people doing bad things.”

• Black Mass opens in UAE ­cinemas on October 1.