At the movies, we hunger for the presence of Robert Downey Junior — the nervy guy in the corner who has taken over the room. He could make just about any picture work, even Joe's Wright's latest, The Soloist – the hokum true-story tale of a journalist (Downey Junior) who rescues from oblivion a homeless, mentally-ill musician (Jamie Foxx). Wright latches onto Downey Junior at the start and doesn't want to let go. The film even starts to mimic him: Downey Junior's LA Times journalist Steve Lopez is up to his usual tricks, all tic and twitch, another personality out of kilter with the world. And the film too, in those initial few moments, follows similar rhythms: all chop and change and quirky editing as if their personalities were merging.

It's an offbeat opening but Wright can't keep up with Downey Junior's nervous energy, the way he moves to the rhythm of his own beat. This is, perhaps, what makes the actor so compelling. If Hollywood exists to impose order on an unstable universe, it operates where real life need not apply: actors are shoehorned to fit stories smoothed into bland fare and clean-shaven rhythms. But Downey Junior knocks his way about a picture as if to say: 'You can't contain me. Life is chaos and order is only an illusion'. His free-wheeling rhythms are what give his films their edge. He is the closest thing Hollywood has now to real life.

Life, noticeably, is what's missing from The Soloist, a film that wants desperately, in its marriage of mental illness and music, to be life-affirming. Unlike Shine, the film The Soloist aspires to be (but with a different ending tacked on), there is no piano. Nor are there the pyro-antics of Geoffrey Rush and his Rachmaninoff Piano Concerto No 3. Instead, we get a cello, some Bach rendered a little soulless, and one Jamie Foxx as the exuberantly-titled Nathaniel Ayers. He stands on the street under a Beethoven statue playing a two-stringed violin (not his chosen instrument), wearing magpie-shiny, high-vis, glitter-strewn, star-spangled clothing. He talks like a runaway train, a stream-of-consciousness pitter-patter. He also tends a shopping trolley full of junk. He might as well be standing on stilts shouting: look at me, I'm mad, I am.

You wonder how Jamie Foxx is supposed to compete with this. His character is so visually loud it is no wonder he retreats into himself. His eyes disappear. He looks nervous. You sense all that jittery talk is actually anxiety about the role, a hope that he can talk his way through it.

Nathaniel is an untreated schizophrenic and a drop-out too: a prodigy who attended the Juilliard school of music. And Steve, whose nose is steered to life between the cracks, thinks Nathaniel is good copy. The drama lies in his becoming involved, and the deepening complexities of helping a man with mental illness.

I wondered might there have been more spark if the roles were reversed: not just because Downey Junior would have toned Nathaniel down, but because it would be more interesting to watch an African-American in the role of the middle-class journalist feeling good about himself because he's helping the down-and-out white man. As it is, though, the film wants to make a point: we travel into the Hades-like slums of LA where Nathaniel lives, and where skin colour is exclusively dark, and Wright's camera recalls the gaze of Preston Sturges in Sullivan's Travels, showing a genuine interest in the faces of the dispossessed. But there's an over-earnestness here too, a patronising rubber-necking that begins to grate.

Wright, who made Atonement, is a volcano of visual energy. It can be a pleasure to watch a director think through his shots so they don't have to be chopped for sense in the cutting room. But he can irritate too, over-thinking images when there is little to say. In one shot, you travel inside a shopping trolley with the point of view of a cello being pushed through a building and it's a pointless distraction. Wright is so busy being quirky, he takes his eye off cliché and lumpy plot. He seems to lack the depth needed to shape material into something unique and original.

Just watch the moment when Steve drives around LA searching for Nathaniel, only for the musician to literally run into his car by accident on a busy motorway. (Of all the cars, in all the towns, in all the world, you had to run into mine.) Or those nauseating close-ups of rapturous faces while Nathaniel plays. (Downey Junior's face is wide with ecstasy; Foxx is concentrating so hard, he hardly moves his fingers.) If only the camera had turned around to my face, gurning with embarrassment. Instead, it trains upwards as digital pigeons climb the sky like doves. I was so distracted by these digital pigeons – could they not afford digital doves? – I almost missed the film's startling message, that music lifts the soul.

For all its sweeping camera movements, the film doesn't move us. It tries so hard to arouse our sympathies that it numbs them instead. Wright forces empathy and feeling instead of teasing it out. Shown here, Nathaniel's mental illness and his music are not enough to make us care. Now that is a tragedy.

The Soloist

(Joe Wright): Robert Downey Junior, Jamie Foxx, Catherine Keener, Tom Hollander. Running time: 117 minutes

Rating: 2/5(12A)