To watch Bjork’s self-lacerating performance in Lars von Trier’s Dancer in the Dark is to understand what costar Catherine Deneuve once said about her: ”She cannot really act; she can just be.” In what many are calling an Oscar-worthy turn, Bjork plays factory worker Selma, desperately striving to save her son’s eyesight even as she’s losing her own. The reports from the set — of conflicts with von Trier, of Bjork running away — only reinforce the power of this emotionally costly but priceless performance. It was Spike Jonze’s video for her ”It’s Oh So Quiet” that caught von Trier’s eye and led him to concoct a dark musical that would exploit her raw talents (Bjork claims it was also the footage of her bashing a reporter for trying to interview her son, now 14). By forsaking his minimalist Dogma ethic to make the wide-screen, violence-racked Dancer, von Trier captures the abandoned joy Bjork shows on a rock stage and trades on her maternal grit. Here, and again on her soundtrack, Selmasongs, she is, as always, living the part of the dangerous, and visionary, innocent.