It might not be miscasting to have William H. Macy play a fussbudget Jewish nerd in Focus. But a non-Jewish nerd who keeps getting mistaken for being Jewish? (It happens whenever he puts on his glasses.) This parable of prejudice and paranoia during the waning days of World War II was adapted from an obscure novel by the young Arthur Miller, and it seems to unfold in some implausible twilight zone of Crayola ethics. Macy’s cowering nebbish, a kind of homefront Bartleby, meets a beautiful, willowy blonde who turns out to have the same mistaken-identity problem that he does. Considering that she’s played by Laura Dern, who preens and sashays like the queen of the jitterbug contest, neither their romance nor their Jew/not-a-Jew ordeal is remotely believable. The director, Neal Slavin, trowels on the irony that the same America that was fighting to defeat the Nazis was, in truth, a hotbed of prejudice. The movie, however, has a topsy-turvy sense of injustice. Its central outrage isn’t anti-Semitism, really, but the far more banal fact that Macy and Dern have to endure the inconvenience of being scorned for what they’re not.