Some visual art prompts abstract questions, like "What is the artist trying to say?" Spencer Tunick's photographs demand a more concrete response: "Are those really people?" and then "How did he do that?" Over the past decade, Tunick has built his name on the basis of his daring photos of nudes in public places, including a record-breaking group shoot of several thousand volunteers in Chile and an assortment of New York City guerrilla shoots, which have gotten him arrested several times. Lying naked in regimented proximity, in one of Tunick's most common modes, the photographer's hundreds of amateur models form a disturbingly homogenous living blanket; sprawled in unposed heaps, in another mode Tunick favors, they evoke the haunting images of mass graves outside Nazi concentration camps. The 2000 HBO documentary Naked States follows Tunick on his late-'90s 50-state tour, during which he sought local volunteers for at least one nude shoot in each state. First-time director Arlene Donnelly chronicles the minor wrinkles in his relatively smooth trip, as he fusses over the difficulty of finding a suitably free spirit in Fargo, worries about being assaulted during a biker rally in South Dakota, and anticipates virtually no turnout at a Phish-related Maine shoot that eventually draws more than a thousand people. Donnelly works in a variety of film stocks and visual styles, and cuts between them for contrast, often documenting the setup of a given shot, then comparing it with Tunick's finished photographs. She keeps Naked States moving briskly without resorting to too much flashy cutting, though some of her sequences do seem suspiciously staged. But she's blessed with an immediately compelling topic, a wealth of attention-grabbing images, and a personable, if not always terribly expressive, central subject. Tunick does speak up on various topics, including the rudeness of nudists, who cheerfully gather for his shoot but then ignore his directions, and who require him to be nude as well. (He complies, but complains that he can't work without pockets.) Donnelly also draws commentary from the models, including a rape victim who found posing therapeutic, an obese New Yorker who faced her own body-image problems through a solo shoot, and Tunick's eventual Fargo subject, a vivacious young woman who ignored the warnings of her friends and posed as a representative step out into a larger, less repressed world. Naked States' only real problem is that once it answers the questions about Tunick's processes, it has nowhere to go; it doesn't address his history, his intentions, or his aesthetic, beyond a single sentence about bodies without fashion and commentaries on war. Naked States answers the most obvious questions, and tells a fascinating and unusual story in the process, but by failing to delve any deeper, it becomes a bit too obsessed with skin and surfaces. It would be gratifying to think that there's more to Tunick's naked obsessions.