It is based on the one-time gangster's own life story, and is, of course, seen through his eyes.
But on top of that director Ray Burdis made changes to the actual tale of gangland enforcement, crime and punishment 'for the sake of entertainment'.
However, John Hannah, who plays Tam McGraw in the movie, doesn't believe the failure to produce the absolute truth in a biopic should get in the way of a good story.
"I have done films in the past, which are said to be historically true, such as The Hurricane," says the East Kilbride-born actor, speaking from New York.
"And you do some research, but then find out things that conflict with the script.
"What happens next is you go into work the next day and find yourself arguing about the point of a scene. And not just with the director, because he then has to approach the writer – and it all gets confused. It's not worth it.
"What I came away with from that was the feeling if you don't like the script, don't do the movie. And I happened to like Ray Burdis's script."
The actor believes a film is also valid as it stimulates debate.
"I'm reading about Lance Armstrong at the moment and his drugs confessions.
"Now, what is written about him and what he has said seem to contain some discrepancies, but the fact he has now come out and is talking means at least the subject is out in the open.
"Where the Paul Ferris movie is concerned it gives us a chance to re-look at some of the issues in the film."
He adds with a grin: "Perhaps there will be some discussion into whether or not the police were corrupt, or if he has perjured himself. Then some light may be shed. But this is a film. It was made to entertain."
John Hannah was not aware of the life and times of the man from Blackhill, who argues he turned to a life of violence after being bullied as a youngster.
Hannah says: "I was a young, blinkered actor trying to make my way in London, and the Glasgow gangster tales have the same sort of cause celebre in London. I read the script fresh, and thought it was a well-told story."
And Hannah thoroughly convinces as McGraw, who was nicknamed The Licensee.
"After the plethora of late 1990s gangster films, it is interesting to see a Scottish film arrive, and in the form of a period piece," he says.
"The way the film presents the story and young Paul's journey and the notion of revenge would suggest the tale of innocence corrupted. And the notion of revenge offers a strong narrative."
It does. But does he feel the Paul Ferris story (he was convicted of gun running) is one of 'innocence corrupted'?
"Look, it's not for some poncey actor living in London to make judgement or even take the high moral ground on that one," he says. "It's Paul Ferris's story and that is what has been filmed.
"If two people see the same event then you could get two versions of that story. This is his. That's what we worked with."
Hannah, a former electrician, was a jobbing actor in London until Four Weddings And A Funeral propelled him into the major leagues of TV and film.
In recent times, he has been in film blockbusters such as The Mummy and TV spectaculars such as Spartacus. Right now, he is in New York working on the Jonny Lee Miller-Sherlock drama Elementary, playing a drug dealer.
So, at 50 his career is going well. "Mmm. There's room for improvement," he says.
"The last few years have been interesting with a lot of different things coming along, playing moustache-twirling Englishmen, whatever."
Will he return to the stage?
"I haven't ruled out theatre," he says. "It's hard work, to do it every night of course. There has to be something about the project that maintains your interest, whether it is the writing, the director.
"But I have young children (two) and I want to spend time with them. If the theatre work allows me to do that, then great. But you're not doing it for the money, that's for sure."
The star offers a chilling portrayal as McGraw, the psychopathic crime boss. We are introduced to his character when he glasses a pub comedian and proceeds to beat him with a savagery unseen outside of the Roman Colosseum, or some of the less savoury Glasgow pubs.
"It's a case of giving what is required and if it's there you tap into it," he says of calling upon his own darkness.
But where did he find this hidden menace within? Did he recall moments of getting up in the freezing cold as a schoolboy in East Kilbride, or going up against torturing teachers?
"I think I thought about going up to meet casting directors," he says, laughing. "That recall was enough to get the bile up."
l The Wee Man is in cinemas now.