From its shaky start, you’d think Appaloosa a first-time effort, but director/star Ed Harris already has Pollock to his credit. The good news is that the film grows in assurance. A revisionist Western in the vein of Unforgiven, it plumbs the gray areas in black-and-white frontier justice. We have Virgil Cole (Harris), a gunman who hires out only to the law, and Randall Bragg (a droll and steely Jeremy Irons), a ruthless rattlesnake with a quick trigger finger and homicidal tendencies. Bragg has paralyzed the town of the title, so the citizenry turn to Cole, setting up the archetypal stand-off and imminent showdown. Despite such conventional elements, the material, adapted from Robert Parker’s 2005 novel, moves in unexpected and rewarding directions. Harris plays it straight up, and the ensemble, among them Viggo Mortensen, Renée Zellweger and Lance Henriksen, bring heft to their roles. 114 minutes | Fenway + Somerville Theatre + West Newton