This crime drama falls frustratingly short of striking. The premise, while based on a true story, is beyond ludicrous – a decent dad (Dwayne Johnson) turns drug trafficker and informant to reduce his college-bound son’s maximum jail sentence – but the movie is shot in such a gritty, class-conscious style that you almost buy it. The characters’ desperation is palpable, and you thrill to some believable supporting turns from Michael K Williams of ‘The Wire’ (scary local dealer), Susan Sarandon (politically ambitious prosecutor) and Barry Pepper (bearded, wizened narc). But our leading man is too physically inflated, too earnest, too much of a walking caricature to be persuasive. ‘Snitch’ cries out for the wiry B-movie stars of old: Robert Forster, a younger Tommy Lee Jones. And a craftier screenwriter would have made his hero’s walk on the criminal wild side a touch more tempting.