Billy Bob Thornton shrugs when I ask why exactly he plays so many Texans. Wearing that perennially you-know-what-eating grin, he takes a drag on his American Spirit. “If you’re from the South, you must be from Texas, right?” he says, imitating clueless film executives.That doesn’t quite explain it, because he’s got the locals fooled too. “Billy Bob’s Texan, ain’t he?” one good ol’ boy asked into the crowd of good ol’ boys and girls. We’re packed into Main Street Crossing, in Tomball, thirty miles northwest of downtown Houston, to hear Thornton sing with his rock band, the Boxmasters. The pearl-snapped fan took Thornton’s Texanness almost on faith, mentioned in between telling tales of decades spent riding bulls and of seeing George Strait before he was…