Writer-director Jon S. Baird (Cass) wastes no time in laying out exactly the kind of irredeemably debauched character we’re dealing with in his twisted adaptation of Irvine Welsh’s 1998 novel of the same name. By the end of the opening act we’ve seen James McAvoy’s Bruce Robertson plan the downfall of his colleagues, repeatedly break the law he’s employed to enforce, abuse his position to sexually assault a minor, spew out a litany of horrendously bigoted remarks, make obscene crank calls to his best friend’s wife (using the voice of Frank Sidebottom, no less), and indulge in some extremely perverse sexual activities - all while subsisting on a cocktail of hard drugs and alcohol. He’s an utterly vile human being, but he's a compelling lead, and we follow him through an increasingly murky period in his life, during which both his behavior and his sense of sanity begin to deteriorate even further.
There’s no room for half-measures, and McAvoy gamely throw himself into things headfirst, delivering a blistering, balls-to-the-wall performance that’s among his very best. Watching the character sink to the very depths of depravity will either be entertaining, hilarious or horrifying depending on your perspective, but those aspects are only superficial. It’s in McAvoy’s portrayal of a man slowly losing his grip on reality that Filth really comes into its own, and Baird’s direction is the perfect complement. Each scene almost feels like it’s being presented through a prism, refracting Robertson’s ever-changing lucidity. So we get periods that are fast and frenetic and others that are hazy and dreamlike. And then there are Robertson’s hallucinations, which at times are just flat out bonkers.
Somehow Baird manages to wrangle all of the competing elements into something approaching a cohesive narrative, and while we’re not asked to like, identify with, or even remotely root for Robertson, we’re asked to stick with him in the hope that he'll find a glimmer of humanity at the end of this squalid tunnel. When the eventual payoff comes at the end of a less-controlled final act, it does feel both earned and satisfying, but there’s an uneasy sense that we’ve followed Robertson further down the rabbit hole than we would have liked. But Filth aims to unsettle, and it does that with aplomb, no matter how dirty it may make you feel along the way.