Filth sells itself as the most outrageous and controversial film of the year and from the get-go it does its utmost to achieve that. Unashamedly crass, vulgar and often completely bonkers, it’s quite the experience and not one easily forgotten. Whether you love it or hate it there’s no doubting it sticks in your mind.

Based on Irvine Welsh’s controversial book of the same name, Filth centres on a bigoted and corrupt Edinburgh policeman who is not averse to indulging in drug taking, casual sexual relationships and “The Games,” the name he gives to the scheming he perpetrates against his colleagues who are all vying for a promotion.

The whole thing working on the crazy level its playing on depends entirely on the central performance by James McAvoy with the likes of Eddie Marsan, Jim Broadbent and Jamie Bell merely providing wonderful punctuation to his personal story. It’s a difficult role to make such a repugnant character – who is saying and carrying out some very nasty things – in any way likeable or entertaining and in the hands of a lesser actor it might have backfired. Luckily McAvoy grabs the bull with both horns, so to speak, committing to the role in a way that keeps him a compelling monster throughout and certainly shows another side to him as an actor we’ve never seen before – think a Scottish Patrick Bateman meets Bad Lieutenant and you’re somewhere close to what he’s doing here.

The film courts controversy almost constantly, with a sort of “let’s see what we can get away with showing” mentality that might turn some people off. Then again, chances are if you’re at all interested in the film then you already have some idea what you’re letting yourself in for and you won’t be disappointed when it fully delivers on its proposed depravity. Its graphicness and sheer conviction should be applauded – it takes a lot these days to get that 18 BBFC certificate and it more than earns those stripes – while its ability to shock and make you laugh at the same time is quite the feat.

It’s commitment to being controversial threatens to wear thin on occasion but director Jon S. Baird, making only his second feature film after the hard-hitting Cass, manages to mould a proper journey out of the madness. The layers of Bruce’s persona are peeled away as it goes on as we learn more and more about why he might be acting this way. It doesn’t quite work when it tries to illicit sympathy – after all how can we truly care about such a twisted individual? – but it never makes that the point and thankfully doesn’t fall into the trap of leaving us on any sort of sentimental or redemptive note. The film walks a fine line between entertaining and grotesque with skill, making for an unpredictable experience that’s destined to become a cult favourite in years to come.

Filth is released in Scotland on September 27th and nationwide on October 4th.