How do two respected investigative journalists feel, now that Hollywood has taken their bestselling book about a notorious gangster and turned it into a movie starring Johnny Depp?

Mostly excited, but also a little nervous about how accurate this cinematic version of history will be, say Dick Lehr and Gerard O'Neill, authors of the 2000 book Black Mass: Whitey Bulger, The FBI, And A Devil's Deal.

Its truth-is-stranger-than-fiction account of how the Federal Bureau of Investigation (FBI) colluded with Boston Irish mobster James "Whitey" Bulger - who was recruited as an informant in the 1970s, but manipulated the bureau into helping him expand his criminal empire - forms the basis of the film Black Mass. It opens in Singapore today.

Speaking at a press event in Toronto last week, Lehr and O'Neill, former members of The Boston Globe's award-winning investigative team - which published a sensational scoop in the 1990s exposing Bulger as a protected informant, embarrassing both him and the FBI - tell Life they are thrilled their non-fiction tale is getting the Hollywood treatment.

"The advantages are to reach a broader audience," says Lehr, a Pulitzer Prize finalist and Boston University journalism professor.

The movie is already attracting Oscar buzz for Depp's prosthetics- aided portrayal of Bulger, whose story has fascinated America.

He went on the run after being outed as an informant and spent 12 years at the top of the FBI's Most Wanted list - second only to Osama Bin Laden - before being caught in 2011. Now 86, he is serving two consecutive life sentences for 11 murders and a host of other crimes.

Lehr admits feeling some nervous- ness about compressing this protracted saga into a feature- length film.

"As journalists, we traffick in facts, that's what we're preoccupied with. But this is a movie, it's a different format and it's an adaptation. And for a story that played out over two decades, this movie, which is two hours long, has to be different.

"As a journalist, there's a little bit of discomfort with that. But the good news is that director Scott Cooper and Johnny and all the actors, they didn't blink at the darkness and the horror that is Whitey Bulger in this story. That would've been troubling."

O'Neill, a Pulitzer Prize winner and Lehr's editor at the Globe, agrees. "It was very gratifying to see that it was faithful to our work. Because it was a very corrupt, difficult period in Boston that needed to be explored," he says.

That this is recent history and many of those who lived through it are still alive, was part of the appeal for director Scott Cooper and the cast, which includes Joel Edgerton as John Connolly, the corrupt FBI agent who helped Bulger, and Benedict Cumberbatch as Bulger's politician brother Billy.

"That was why I was so drawn to it,'' says Cooper, who directed the Oscar-winning 2009 drama Crazy Heart. "Too often, I find myself enraptured by gangster and crime films, but they're largely fictitious. So when I had the opportunity to tell such a compelling story, from two men who put their lives on the line to chronicle this for 37 years, it was too intoxicating to pass up."

The stakes involved in telling this story have been high, with Lehr and O'Neill's investigative reporting for the book putting them at odds with the Irish mob and Italian mafia in Boston as well as the FBI.

Asked which of the three behaved most menacingly towards them, O'Neill gives a somewhat surprising answer. "Well, ironically, the most threatening episode was with the FBI; they were trying to push us off the story. They basically said, 'You don't know who you're messing with - Whitey Bulger will kill all you guys.' So that was a little daunting."

Lehr adds: "Think about that - the FBI was the messenger of the threat."

Cooper, 45, jokes that the bureau could well be irked by his movie as well. "Something tells me I'm going to get audited for the next four or five years," he says.

Edgerton, 41, who directed and starred in the recent thriller The Gift, admits he felt a bit of the fear factor while promoting the film.

He had read that John Morris, one of the corrupt FBI agents who protected Bulger, had received a threatening phone call from Bulger while the latter was on the run "and that night, Morris had a heart attack".

" I remember the other day saying in an interview something about Jimmy (Bulger) being a psychopath. Afterwards, I thought, 'Oh, I hope that doesn't come back to haunt me.'"

There was also the pressure on the actors to do a convincing version of the distinctive Boston accent, particularly from the historically Irish and working-class neighbourhood of South Boston where the Bulgers lived.

Julianne Nicholson, 44, hails from Boston herself so it was easy for her, but she says that "people from Boston will criticise the accent of people from Boston" in movies.

"Before I did this, my aunt was like, 'Don't mess it up. Laura Linney in Mystic River, the worst!' So even my own family was putting pressure on me."

Tough-talking Bostonites did not let the Australia-born Edgerton off the hook either. He says: "I came to America through the Canadian border and the guy stamping my passport was from South Boston and he knew I was an actor and I told him what I was about to do.

"He kind of jokingly, but not really, said, 'You'd better not f*** it up.'"

Edgerton and Cumberbatch, 39, worked with a dialect coach and pored over audio and video recordings of the people they were playing.

But co-star Cochrane, 43, who plays Bulger's henchman Steve Flemmi, said no to the dialect coach. Instead, he opted to do more hands-on research by venturing into some of the rougher parts of South Boston.

"I asked a Boston guy to take me to Southie and asked him where the local guys hang out. He said, 'Well, there's that restaurant over there, but I don't know if you want to go in there'. I'm like, 'Drop me off' and I just sort of hit the pavement."

Between that, digging up old footage and reading the books by Lehr and O'Neill, he became obsessed about finding out the smallest details.

"Because Bulger is a very important mythological figure, whether good or bad, and it's like this folklore that has now spiralled into whatever it is. So there's a lot of pressure that you will be held accountable if you don't get it right."

• Black Mass opens here today.