“ Black Bag ,” an elegant, quick-witted new thriller that finds Cate Blanchett and Michael Fassbender playing married spies, isn’t the pair’s first time together on screen. That honor goes to 2017’s “Song to Song,” one of Terrence Malick’s most intangible and head-scratching offerings — though the two stars can barely recall that experience.

When George is handed a list of possible traitors, including Kathryn, he immediately sets out to discover whether her wife is lying to him. After an extensive game of cat-and-mouse, which at first appears to indicate Kathryn is playing both sides,