With movie-star good looks and a knack for alternating hushed and forceful harmonies, it's no surprise that Joy Williams and John Paul White of the Civil Wars have been playing to sold-out crowds on their cross-country tours (including this date at the Fox Theatre). The act occupies the space between polished country and hand-hewn folk, with Williams and White singing as hard as any Nashville up-and-comer, but their simple arrangements keep the songs rooted in modesty and simplicity. The slow-burning "Poison & Wine" proves that the pair can imbue a piano ballad with soap-opera-worthy theatrics. Secret admirers of Lady Antebellum will find much to like in the Civil Wars, as will fans of the Swell Season's fraught odes to rocky romance.