Johnny Depp has revealed he felt "a tremendous amount of responsibility" to portray real-life gangster James "Whitey" Bulger.
The Golden Globe-winning actor, star of films such as Pirates Of The Caribbean, The Lone Ranger and Edward Scissorhands, admitted he felt some pressure to do the role justice as the film premiered at the Toronto International Film Festival.
"When you're playing a fictional character, there's room for... you can kind of stretch it out into strange places. When you're playing someone who either existed or exists, there's a tremendous amount of responsibility, at least for me," he said.
"No matter who they are, whether they're deemed good or bad, you have a responsibility to that person because it's their life. You also have the responsibility for history and truth to some degree."
Depp plays Bulger - a convicted murderer who is serving a life sentence at United States Penitentiary Coleman II in Sumterville, Florida - in Black Mass, which also stars Benedict Cumberbatch, Joel Edgerton and Kevin Bacon.
The film, directed by Scott Cooper, is based on Dick Lehr and Gerard O'Neill's 2001 book Black Mass: The True Story Of An Unholy Alliance Between The FBI And The Irish Mob.
The 52-year-old actor said Bulger - a member of the Boston mob known as the Winter Hill Gang - declined to meet him in person.
He explained: "The first thing I did, I contacted Bulger's lawyer Jay Carney to request the opportunity to meet James Bulger, to hear his take, certainly... and to be able to study him.
"About a week after I made the request, I got a message from Mr Carney that said, 'Jimmy respectfully declines as he is, as you can imagine, not a great fan of the book', nor any of the books. He would never put his client into any sort of weird situation but he was very helpful with regards to the heart of Bulger, the heart of the man."
Depp - who arrived with wife Amber Heard at the film's Canadian premiere - said he wanted to show a "human" side to Bulger.
He said: "My intention was not to go out and create someone who is evil, because I don't think any of us wake up in the morning, shave and brush our teeth and think, 'I'm so evil, I'm so horrible'.
"I approached James Bulger as a human being who is multi-faceted and had a side to him that was human, loving and all that. And then he's in this business - certain businesses out there, where the language of that work is violence, so that's the only way I could approach it."
The star also joked about how box office success is important to him.
"I'm gagging for it. It's been the thrust of my interest since day one, when I was about 19. That's all I care about. Twenty years of failures will do that to you," he quipped.
He added: " If you feel you've done service to the director, to yourself, to the author, then to me, that's a success. Box office, that's something else and it's none of my business. Once I've wrapped on a film, I say goodbye to the character."
Black Mass opens in the UK on November 27.