Interviews: Cast and crew of Cannes hit Un Prophète: 'Audiard is like a sculptor' guardian.co.uk

Du Rififi Chez les Hommes was the full title of Jules Dassin's classic tough-guy thriller from 1955 – aggro among men. Here, it is more a case of Rififi Among Men and God alike, in a blisteringly ­powerful prison-gangster picture from the French director Jacques Audiard. It comports itself like a modern classic from the very first frames, instantly ­hitting its massively confident stride. This is the work of the rarest kind of film-maker, the kind who knows ­precisely what he is doing and where he is going. The film's every effect is ­entirely intentional.

Newcomer Tahar Rahim plays Malik El Djebena, a young Arab guy about to start a six-year stretch in prison for what appears to be violence against police ­officers. He is a 19-year-old petty ­criminal, and this is his debut in adult detention. Malik is very frightened, cringing almost visibly into his clothes on walking the grim corridors of jail, and into his nakedness when he is inspected by medical officers.

On what is apparently his very first day in the exercise yard, Malik's ­vulnerability and his very blankness attract the hooded eye of César, the Corsican mobster with the guards in his pocket – incomparably played by Niels Arestrup. César needs someone to whack a fellow prisoner, who is about to incriminate his ­associates on the outside by turning state's ­witness. Surrounded by his thuggish courtiers, César curtly summons bewildered small-timer Malik and informs him that he must kill this switch, or be killed himself by César's lieutenants. He will be given ­instruction on how to do the job, and protection from César's crew for the rest of his term. No ­arguments: Malik is "in", a murderer. There is no way out.

Trembling Malik now finds himself in a terrifying, almost Greeneian dilemma. Should he refuse? Should he simply ­submit to death rather than ­become a murderer? The plan is that Malik must kill his victim, Reyeb (Hichem Yacoubi) with a razor-blade concealed in the roof of his mouth. Pretending to offer him a blowjob in his cell, he must work it out with his tongue and push it forward between his teeth while his face is invisibly at crotch-level, and then stand up and cut the man's throat. The scenes in which Malik must practise doing this in front of the mirror, retching and spitting blood into the sink, are the stuff of pure, scalp-prickling fear: I just can't remember ­being so tense in the cinema.

This nauseous forced deal between Malik and César appears to become the basis of a strange, unknowable spiritual bargain between Malik and God – or does it? Audiard makes the haunted Malik the centre of an internal crisis, part ­psychological, part ­supernatural. The terrible unsought burden of ­assassinhood transforms him into a grotesque, parodic "prophet" and the agent of César's downfall. Intent on self-betterment, ­Malik takes classes, learns Corsican-dialect Italian and, to the contemptuous disgust of the Muslim prisoners, becomes the Corsicans' Uncle Tom-ish servant boy.

But poker-faced Malik has big plans; he is rising through the ranks – and laws from the new Sarkozy ­government about repatriating Corsican ­prisoners away from mainland French jails now leave César exposed, with no ­bodyguards. ­Malik, whom César fears and suspects more than anyone, is his Quisling ­nemesis, his only companion, and the son he never had or wanted to have.

Audiard has created a long, involved, relentlessly brutal but gripping and thrilling picture; it has the rangy, ­anecdotal feel of something drawn from real life, but its realism somehow ­accommodates an eerie supernatural shimmer. Malik has visions which are partly, but apparently only partly, ­explicable as trauma. The sweat and the machismo are very familiar from the French crime genre, which was ­revived only ­recently in the 70s-era Mesrine films. The passing of contraband, the ­defiant songs and shouts and ­burning garbage being flung from the high ­courtyard walls surely also ­summon up ­memories of Pontecorvo's The Battle of Algiers. But the movie has its own ­muscular originality.

Niels Arestrup is all too plausible as the jail gang-boss, coolly ­proprietorial with prisoners and guards alike, ­sporting a mask of impassive disdain through which world-weariness and fear gradually surface. But Arestrup and ­Audiard have found something new in this classic persona. What we see etched on César's face is pathetic ­loneliness and the horror of dying alone in prison. His weakness and Malik's future strength – this is the emotional ­fulcrum on which this tremendous film is ­structured.

Arestrup was outstanding in ­Audiard's previous film, The Beat That My Heart Skipped – also, intriguingly, in a tense, mutually resentful relationship with a younger man – but here he brings out new strains of desolation. Rahim, too, is a tremendous casting find for Audiard. The film returns us to what should be the biggest cliche in the book: the prison film, with its cells, its shouts, its ­corrupt guards, its boxes full of ­prisoners' heartrendingly meagre ­personal effects. But Audiard also revives the hidden source of our ­fascination with prisons. They are places of violence and fear, but also of ­paradoxical freedom – freedom from the ties of outside lives. They are places you can remake yourself, for good or ill, ­hellish furnaces in which you can smelt a new identity.