For there can be few more outrageous anti-heroes in recent times, on page or on screen, as Irvine Welsh's Machiavellian, misanthropic, coke-snorting, bipolar and irredeemably corrupt Edinburgh cop.
Robertson is a dream part, but not an easy one to crack. We need to be horrified, but also compelled and a little sympathetic; if he is simply a monster, we won't stay with him. There is nothing to fear here. Filth is a scabrous, fabulously funny yet surprisingly moving adaptation, whose political incorrectness is too deliciously wicked to take seriously. And in James McAvoy's Robertson, it features what I genuinely believe to be one of the best ever performances in a British film.
When a young man is brutally murdered, Bruce's boss, Chief Inspector Toal (John Sessions), needs speedy results, suggesting to his team that whoever cracks the case will win promotion. Bruce sees his opportunity both to climb the ladder and win back his estranged wife and daughter. Our dastardly hero is the front runner, at least in his eyes. Nevertheless, being a twisted fellow, he proceeds to spend more time manipulating and undermining his colleagues - sleeping with their wives, turning one against the other - as he does trying to solve the murder.
Bruce refers to his office shenanigans as "the games". Another project in which he combines advancement with devilment takes place at the Masonic Lodge, where he befriends the naïve accountant Clifford Blades (Eddie Marsan), promising 'Bladesey' a good time while stalking his wife Bunty (Shirley Henderson) with obscene phone calls.
Bad behaviour can be a wonderful guilty pleasure for a viewer, and some of Bruce's Christmas jaunts bring to mind Billy Bob Thornton's bad-taste classic Bad Santa. But just as Thornton's character experienced an eventual mellowing, so Bruce has his more vulnerable side and a sort of comeuppance. Director Jon S Baird, who has also written the adaptation, subtly feeds our awareness of what lies beneath Bruce's bravado - the cop's visits to the psychiatrist Dr Rossi (Jim Broadbent) and the creepily seductive visions of his wife suggesting the mental illness and despair fuelling his addictions.
Baird structures the film very well, alternating between grim reality and wild fantasy, between Bruce's own perspective and, slowly, that of others who are less the victims he thinks they are. The result is a veritable helter-skelter, rich in language and peppered with surreal, visual coups de theatre, including Bruce and Bladesey's drug and sex holiday to Hamburg, a laugh-out-loud musical appearance by David Soul, and the increasingly bizarre fantasies involving Dr Rossi - with Broadbent brilliant as a character whose insights effectively replace those offered by the tapeworm of the novel.
Broadbent's is just one in a roster of perfect supporting performances, including those of Marsan, Henderson and Jamie Bell, as Bruce's trainee bad boy. But the key to the film's success is undoubtedly McAvoy. Red-eyed and ginger-bearded, with his trademark cheek cranked to the max, the actor captures the character's venality, waggish delight and pain, often all in the same moment. It's a raw, rude, complex and truly great performance.
Welsh's work has had its fair share of film and TV airings, but it has not been this well represented since Trainspotting.