This is another film adaptation of an Irvine Welsh novel that was referred to as "unfilmable", although when reading the book when it first came out I for one was struck by the tightness of the narrative and the cinema-friendly focus on a single protagonist.
The antihero in question is Bruce Robertson (James McAvoy), a dodgy copper trying to make the most of a promotion opportunity by ruining his rivals through a series of cruel intricate schemes. Meanwhile, his mind is deteriorating, and he's haunted by flashbacks, waking dreams, and humanoid livestock. The film is fairly faithful to the source, and the changes (including some understandably blunted edges) are down to the different artform.
Irvine Welsh has said that McAvoy's performance is better than De Niro's in Taxi Driver. I don't think this is a suitable comparison. Scorsese's seminal feature was about a post-traumatic depression, whereas Jon S. Baird's film is more manic. For me, the film Filth most resembles is A Clockwork Orange. Like Kubrick's masterpiece, the entire aesthetic is informed by the subjectivity of the central character. And there are subtler nods: the use of classical music, the bleached windows, Jim Broadbent's reinvention of the Deltoid character (a probation officer then, a psychiatrist now), and the visual reference to 2001: A Space Odyssey.
Before the film's release, I wasn't convinced by the casting of McAvoy, but after watching it I can safely say he's transformative - to capture such bipolar savagery and the fear in a single facial expression is the sign of a special performance. The supporting cast provides a colourful blend of caricatures. Jamie Bell, Eddie Marsan and Imogen Poots all make an impact in the few moments when McAvoy isn't dominating the screen.
For me, the dud notes concern the tone of the film. Sometimes Baird's shifts between the schizoid black comedy of Robertson's outbursts and his introspective guilt about his past are so sudden and sentimental that their capacity to convince is lost in the (lack of) transition. Part of this is down to Clint Mansell's disappointingly soft score, whose tinkly piano and lifeless strings often feel incongruous, more awkward than deliberate.
But these minor issues don't detract from a powerful, funny, and finally moving depiction of mental disintegration. To say that it's the best Welsh adaptation since Trainspotting may not be saying much - so I'll say instead that it's a very good film in its own right.