In “August: Osage County,” we enter the story (based on the Tracy Letts play), of the appalling Westons who live in Oklahoma. The disappearance of the patriarch, a poet and a drunk (Sam Sheppard), brings together the members of this spectacularly dysfunctional family. In the stifling heat, they claw and tear each other to pieces, they cuss, yell, and throw at one another’s face awful revelation after awful revelation. Intelligent writing and great ensemble acting make “August” fascinating.