“Why would you ever take in a stranger?” The question posed by FBI agent Nancy Fisher comes late in The Imposter. Bart Layton’s fascinating film, recently short-listed by the Academy Award for Best Documentary, traces the case of Frédéric Bourdin, one of the stranger strangers you might imagine. In 1997, the 23-year-old called the police in Linares, Spain, pretending to be a tourist who’d discovered a traumatized boy in a phone booth. When the police arrived, Bourdin pretended to be the boy; taken to the police station, he was left alone in an office where he found the name of a missing boy from San Antonio, Texas, Nicholas Barclay. He then pretended to be Nick, whose family, desperate to find him, took in.