Standing in the shadow of the smoldering rubble of Ground Zero in World Trade Center, a pious ex-Marine (Michael Shannon) finds the blessing of limited visibility. ”It’s like God made a curtain with the smoke, shielding us from what we’re not yet ready to see.” A similar principle is at work in Oliver Stone’s film, which tilts heavily toward the uplifting compassion of that September day. Two Port Authority cops, trapped beneath the collapsed towers, find strength in their devotion to their willful wives, but their stories are isolated from the overall tragedy of 9/11 and could easily pass for a Lifetime movie about a San Francisco earthquake. Yet the DVD remains valuable for two reasons: As a portrait of the complex Stone, commentary and Q&A’s capture his mission (”[It’s] the love story that I always wanted to do to please my mother”) and his ambivalence about his childhood (”I don’t sentimentalize it. [New York City] was never a place of great warmth”). ”Common Sacrifice,” a harrowing doc, pulls back the film’s veil of caution to examine the gruesome injuries to taciturn Sgt. John McLoughlin and wide-eyed rookie Officer Will Jimeno, while introducing the men who risked their own lives to save them. When emergency medics describe McLoughlin’s dire condition and the plan to amputate his legs in order to extract him, the documentary delivers the terrifying urgency the film dodges.