The novel "Filth" follows logically in Irvine Welsh's Edinburgh works, all strong on Scottish identity, language and culture. His first of these novels, "Trainspotting", was adapted into a masterpiece of a film by Danny Boyle in the mid 1990s, a film that really captured the time period in question with its particular problems and youth culture. Filth isn't quite as relevant, and writer/director Jon S. Baird is very obviously inspired by both Boyle's groundbreaking work in Trainspotting as well as Stanley Kubrick's in A Clockwork Orange (the scenes with Dr. Rossi are practically nicked from the Mr. Deltoid scenes), but if you're going to find inspiration in previous works, why not from the very best.
There are differences between these two Welsh adaptations, however. First of all, our protagonist this time isn't an adventurous heroinist, but rather a completely amoral and misanthropic career policeman with psychopathic tendencies. He is played by James McAvoy, who looks aptly unkempt here and really makes Bruce his own. His work is more reminiscent of Christian Bale's work in American Psycho than Ewan McGregor's in Trainspotting. And much like Patrick Bateman (or Alex De Large for that matter), Bruce's lifestyle and conduct is so outrageous that it's funny − ingeniously funny, actually. And director Baird knows how to make us revel in this dubious form of fun, without allowing us to lose our footing completely.
In other words: Filth is wildly over the top and stylistically excessive. But it works, thanks to Baird's ability to balance it all against the more serious undertones. The stylized presentation of otherwise horrible actions puts them into perspective, and through these stylistics, we're able to get inside Bruce's head (hence illness) before the plot brings us there. We're able to live and breath Bruce, with everything that implies in terms of fun, pain and shame. And so does McAvoy, who demonstrates his versatility and range with this part.