So Johnny Depp, best-known for playing imaginary characters, how was it playing the very real, very nasty Boston gangster James "Whitey" Bulger in Black Mass? Much of a change?
"You mean the Mad Hatter never existed? Or Willy Wonka?" the actor teases.
"Seriously, when you're playing a fictional character you can stretch it out into all kinds of strange places which I've taken a lot of heat for," he chuckles. "But when you're playing someone who existed or exists there's a tremendous amount of responsibility to that person, whether they're deemed good or bad, because it's their life. You have a responsibility to be as close to the truth as you possibly can."
Depp had gone the real route before, in 1997's Donnie Brasco, playing Joe Pistone, an undercover detective who went undercover as Brasco to infiltrate the mob.
"I was fortunate to spend time with Joe Pistone and I was able to make a lot of changes in the screenplay that weren't remotely true," Depp explains.
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Then in Michael Mann's Public Enemies (2009) he portrayed John Dillinger, the Depression-era gangster gunned to death at the age of 31. "I spoke to his last living relative, his sister, and she said he was the sweetest, funniest guy in the world. I actually thought so," Depp Bulger, now 86, was very much alive when Depp came to prepare for Black Mass. But the crook, who had evaded the law until his 2011 arrest, wasn't about to talk to any movie star.
Depp relied on Bulger's defence lawyer and friend Jay Carney to keep him on track.
Though it's been said that in South Boston in the 1970s certain lawmen and certain criminals were indistinguishable, Bulger's look was unmistakable, his receding hairline emphasising his terrorising blue-eyed gaze.
"The look was extremely important to me, it's really everything in terms of finding Jimmy Bulger, so I wanted to capture it as much as is humanly possible," Depp says. "My regular makeup artist Joel Harlow did brilliant work. He sculpted Bulger's face on a cast of mine and we did about five or six tests until we got to that place where it felt like Jimmy Bulger - much to the chagrin of the producers and money people. It was a couple of hours of makeup every day."