A Dangerous Method, David Cronenberg, 100 mins (15)

Do go through, Fräulein Knightley – the doctor will undress you now

It couldn't be a more mouth-watering prospect: David Cronenberg and Sigmund Freud, face to face at last!

For years, the Canadian auteur stood unchallenged as the most overtly Freudian of all film-makers, an explorer of the spectacular ways in which the Repressed was wont to Return, to wreak havoc on minds and bodies. But Cronenberg has lately remodelled himself as a classicist, a master of understatement. If you're hoping for mutant tentacles flailing over the couches of old Vienna, A Dangerous Method is not your film.

But you don't have to be a Cronenberg cultist to find this piece surprisingly staid. It's scripted by Christopher Hampton, after his play The Talking Cure and John Kerr's book A Very Dangerous Method. Set between 1904 and 1913, it's about the relationship between Swiss pioneer Carl Jung (Michael Fassbender) and his patient Sabina Spielrein (Keira Knightley). While treating Spielrein's hysteria, Jung contracts a serious case of transference and starts an affair with her, which largely takes the form of decorous, almost comically solemn spanking sessions. Meanwhile, Jung's relationship with Freud (Viggo Mor-tensen) falls on rocky ground – partly because Freud disapproves of his acolyte covering up his misbehaviour, partly because of Jung's zanier ideas.

These emerge in a scene in which Jung jumps at the cracking sound of a shelf in Freud's study, just after the latter has warned him against dabbling in mysticism. Jung knew the crack was going to happen, he announces. ESP – or just regular Viennese carpentry? Jung declares it a "catalytic exteriorisation phenomenon", at which point Freud looks ready to call for a straitjacket. Where has Jung's bizarre notion come from? Out of the blue, because we've not heard him harp on such eccentricities before, and the question never arises again.

In fact, Jung hasn't been the same since tangling with disturbed analyst Otto Gross, who believes in screwing your patients as a matter of good health. Gross – drolly played by Vincent Cassel in bristling beard and tweeds – is a Lawrentian advocate of unleashed libido last seen rogering a nurse against a ladder, before clambering up it to make his exit.

Disappointingly, the film is less about the extraordinary Spielrein – who became a pioneering analyst herself – than about the oedipal clash between master and acolyte. Freud and Jung are two hyperintelligent men constantly talking at cross-purposes, and the driest comedy revolves around them. After Jung questions Freud's insistence on interpreting everything through sex, Freud, analysing a dream of Jung's, muses, "This log – I think perhaps you should entertain the possibility that it might be your penis" (a suggestion that may have special resonance for admirers of Fassbender's recent performance in Shame).

Mortensen is terrific – sardonic and urbane, and although that famous cigar barely leaves his mouth, you never feel he's doing the standard cartoon Freud. And Fassbender gives the well-heeled Jung an almost constipated respectability that sits with telling incongruity alongside his sexual investigations. Knightley, though, I'm not so sure about. She's firing on all cannons (excuse me if that seems a tad phallic) from the moment she arrives chez Jung, rolling her eyes, contorting herself, and violently thrusting forth that extraordinary chin – a performance reportedly modelled on the body language of hysterical patients of the period. But while Knightley is clearly pushing her talent to its outer limit, she seems to do it only in one direction; in Spielrein's calmer moments, her delivery is flat, tensely staccato and marked by a distractingly weird American-Russian accent. It's a turn that sits oddly among the other players' naturalistic styles.

This is a very stagy film, but Cronenberg doesn't make the most of the theatricality. Mostly, it feels like a succession of sober stagings of verbose, often academic dialogues: "I was interested in what you said about monotheism, that it came historically out of some kind of patricidal impulse" (by Jove, they didn't call it the talking cure for nothing).

Ultimately, it's hard to know what's more important here: a historic rift for psychoanalysis, or a human drama about flawed individuals? And how exactly is the method "dangerous"? Spielrein seems to have benefited from it, Freud and Jung went on to further glories, while it's Jung's wife Emma (a pallid part, given intensity by Sarah Gadon) who came off worst. But the film never really establishes the stakes of the drama. No doubt Cronenberg is expressly flouting a tradition of films about psychoanalysis that are excessive and overtly dream-like. But this decorous, if visually beautiful, piece feels too repressed to breathe.

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