WHEN AN ACTOR has a breakout performance, as Oscar Isaac did in 2013's Inside Llewyn Davis, you suddenly expect to see him everywhere. Isaac's great accomplishment, however, has been that we never see him doing the same thing twice. He ended 2014 with A Most Violent Year, in which he played a struggling heating-oil entrepreneur in '80s New York—about as far from Llewyn Davis, the '60s folksinger, as you can imagine. His jam-packed 2015 has included a scene-stealing role as a tech billionaire in Ex Machina and, now, Show Me a Hero, a six-hour HBO miniseries from The Wire's David Simon, which premieres August 16 and in which Isaac plays Nick Wasicsko, the onetime real-life mayor of Yonkers, embroiled in a fight over segregated housing. And, oh, by the way,…