Oftentimes authors whose books are adapted into movies are left to sit at home and simmer as directors make the rounds saying how their “reimagining” of the work was necessary to make it a better cinematic experience, blah, blah, blah, while every fan of the novel knows exactly how it was butchered.

Yet, here in front of me, in the same room, are two Scotsmen — director Jon S. Baird and author Irvine Welsh — contentedly discussing the book and movie versions of “Filth,” with nary a hint of venom in the air. They sit there chuckling about the Shinjuku fembot bar they were drinking at the night before. What gives?

“It would have been easy to do this book really badly,” says Welsh, “as a bad-behavior-type movie without any depth to the character.” Indeed, Welsh’s novel was optioned shortly after publication in 1998, but went through a decade of disputes and development hell before Welsh was introduced to Baird via a mutual friend. “He knew the book better than I did,” says Welsh, who was convinced enough by Baird’s passion for the project to get personally involved in pushing it through. Baird, however, jibes him, saying, “He just wanted a drinking partner, that’s all.”

The movie centers around the character of Bruce “Robbo” Robertson (James McAvoy), a fantastically debauched detective who slowly begins to lose his grasp on reality via some combination of bipolar disorder, pharmaceuticals and cocaine addiction. Baird, while a fan of Danny Boyle’s adaptation of Welsh’s novel “Trainspotting,” realized early on the book would need a different, darker approach, and turned to Stanley Kubrick’s “A Clockwork Orange” for inspiration.

“I felt it was a big reference because it had this anti-hero, a despicable character, but there’s a certain charm to him as well; you can’t help being seduced by him, in a way,” says Baird. “And I’ve always loved the way Kubrick set up his movies based on quite symmetric production design, how everything was heightened and larger than life.”

I ask Welsh if he recalls the initial idea for the Bruce character: “Just a guy who’s kind of lost,” says the author. “He’s no longer young, no longer married, and the only thing he’s got is his job. He’s pushing everything into that, ’cause everything else is gone. A lot of people, when they get into their 40s they get to that desperation point where it’s like, is this all there is? There’s two things you think: You need to get an advance in your career, the last chance to make it big. But it’s also your last chance to have that last fling of youth, to have those sexual encounters and partying and fun that you wanted to have. The desperation really comes home.”

Originally, Welsh hadn’t even planned to make Bruce a cop; an early draft had him as a council worker, but, notes Welsh, “It was a little boring. If you have a crazy council official, maybe the rubbish doesn’t get collected, but if you have a crazy cop, there could be serious trouble. It just had more dramatic potential.”

Such changes are not uncommon for Welsh, who notes that “all my stuff tends to be character driven, so you kind of find the character as you write the book. I think when I started off writing ‘Filth’ I just had him being a bastard to everybody in all these different set pieces, but you start to ask yourself, ‘Why is he doing this? What is it that’s driving this?’ So then you start going into his back story, where he comes from, what are the kind of conflicts in his life that determine his behavior that he’s not been able to resolve. So you start building the character up in layers.”

Baird jumps in, saying, “I remember walking through Dublin with (Welsh) after our first boozy lunch together, and I asked him the same question: “How do you get these characters?” And he said, ‘The first thing I do is I apply three rules: where they stay, what they play and who they lay.’ ”

McAvoy’s performance is an intense one, and I ask the director whether the actor had any Robert De Niro-style method acting tendencies, like staying in character all day even when the cameras weren’t rolling. “He’s not methody at all,” says Baird. “The only method kind of thing he did was he drank a lot of whiskey. He told me he was drinking half a bottle each night. But it wasn’t to do his scenes drunk: A lot of that was to physically look like sh-t, and feel like it as well. You know, that horrible sort of resentment where you get a little bit niggly and stuff. But he just went straight in with two feet; he set this level of professionalism and everyone just went up to that level.”

One meaning of “filth” is British slang for the police, and the original cover art for the book was controversial — a pig’s head wearing a bobby’s helmet. (The film posters do much the same, with an image of a uniformed McAvoy riding a giant sow.) Surely, I ask Welsh, this book did not make him popular with the police.

“When it first came out, the police raided two different shops in York and Southampton,” explains the author, “to seize copies of the book because it was ‘offensive.’ (Around then) I got involved in a drunken escapade and ended up in a jail down in Exeter, and I thought I was gonna get a kicking in the cells by the cops, but they came in again the next day with copies of the book for me to sign. (Laughs.) I’ve signed copies for cops at football games, and the rank-and-file cops will tell me, ‘We’ve got a guy exactly like that — we call him ‘Robbo.’ ”

For a chance to win one of three copies of the Japanese translation of the original “Filth” novel, visit jtimes.jp/film. The deadline is Nov. 25.

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