Laura Poitras' new documentary, "Citzenfour," is the kind of stirring thriller that David Fincher or Paul Greengrass could have turned into a feature film. Part of that feeling comes from the way Poitras films Edward Snowden's interviews with Glenn Greenwald, clandestine meetings that bubble with creeping paranoia and mounting tension; part of it comes from the music, which sounds like the most Trent Reznor-y score this side of "The Social Network." As it turns out, there's a reason for that second...