James Franco admits he had "mixed feelings" about accepting the role of Christian Longo, a man who was put on the FBI's Most Wanted Fugitives list after murdering his wife and three children and fleeing to Mexico.

There, Longo assumed the identity of the disgraced New York Times journalist Michael Finkel, and when he was eventually arrested and extradited to the US, he asked to meet the writer.

True Story, a film co-produced by Brad Pitt, is based on Finkel's bestselling memoir of that meeting.

"By the time Longo and Finkel [who'd fabricated interviewees in a piece about the African slave trade] met, they'd both hit bottoms of their own, and the private space that created was, for a while, a positive thing for both of them," says Franco.

"They could confide in each other in ways they couldn't confide in anyone, and I think they helped each other through that time. It's just that Christian Longo has a way of creating his own mythology about himself, and he used Finkel to help him rewrite his own story."

Despite his reservations about the character, Franco was keen to work with the British theatre director Rupert Goold, who was taking the helm on the movie.

"I saw his stage production of Macbeth in London and I'm a big fan," says the 37-year-old actor, whose big break was in 1999, playing Daniel Desario in the cult TV series Freaks And Geeks.

It's not the first time Franco's played a character who's based on a real person, but his approach has always been dependent on the movie and the person in question.

"People know what James Dean sounded like, and what he looked like and how he moved, those are all keys to his character, and capturing his outward behaviour was vital," says the actor, who won a Golden Globe for his portrayal of the late Hollywood icon in a 2001 biopic.

It was the same story when it came to playing poet Allen Ginsberg in 2010's Howl ( "His look and sound is very specific"), but with 127 Hours, the 2010 film which told the story trapped canyoneer Aron Ralston, who ended up cutting off his own arm after becoming trapped by a rock, director Danny Boyle didn't want Ralston on set every day.

"That performance wasn't about capturing the nuances of Aron's physical behaviour. It was about capturing the experience he went through, in as honest a way as possible," says Franco, who received an Academy Award nomination for the role.

"Here, again, I feel that Rupert's approach is not to capture Longo's behaviour to a tee, as much as to try and capture the weird psychology of the kind of person that could do something like this, and then behave the way he has since then," he adds.

Franco has never had any interest in meeting Longo.

"This is the one case where I don't want to meet the guy. I feel like just doing this movie is almost too much," notes the actor, who was born in Palo Alto, California, and also has a string of producer, writer and director credits to his name.

"He killed his family, and now a movie's being made about him that stars Jonah Hill [as Finkel] and myself. If I was making a Werner Herzog documentary about death row, yeah, I'd go and meet him. But because I'm playing him, I just want to separate myself."

While he's managed to find the humanity in dubious characters in the past, "with Christian, I don't find any need to humanise him. He's worst human being I've ever played. I hate this guy", Franco admits.

Instead, he decided to "play him as he saw himself".

"Playing it that way, combined with the knowledge of what he did, will make him a very terrifying person.

"Longo's all surface and appears to be a regular guy, charming and nice, so I feel like I can play those sides of him. Plus, there was some disconnection between what he did and how he saw himself, so I think in this case, it's OK that I don't feel as attached to this world as my other roles, because I feel he wasn't attached to himself."

The upshot was that Franco never took the role home.

"That's the weird thing about this," he says, recalling filming Longo's testimony scene during the murder trial. "I think it was five pages of dialogue, and when I got home that night, I felt that if somebody had asked me if I did a big speech that day, I wouldn't have remembered it.

"Normally with a role, you want to connect the character to your own emotions, but this guy doesn't have any emotions, so I just kind of walked through it."

Now the film's being released, the hardest aspect for Franco to accept is that the narcissistic Longo, who was sentenced to death in 2003, is achieving something akin to celebrity status.

"We're certainly not celebrating him or giving the other side to his story or anything like that, but even though we make him the villain of the piece, he's almost getting what he always wanted," he notes.

"It's that weird, insidious thing that happens when somebody does a horrible act and yes, they are punished by the law, but in the media they're given attention. They're made into celebrities.

"I don't want to give him any kind of positive reinforcement whatsoever," says Franco. "He's done nothing but murder his family. He's a sociopath. I just hope he finds peace in his soul."

:: True Story is released in cinemas on Friday, July 17