This song is about how when we think we know somebody, we often hardly know that person at all. "The Stranger" is the collective for the unknown things about people we know: "Although we share so many secrets, there are some we never tell."
According to Joel, the song can also relate to how we don't always know ourselves. He says that his halfhearted suicide attempt at age 21 (he tried to end his life by drinking furniture polish) was an inspiration for the song, as it revealed a dark side of his personality that wasn't readily apparent.
Adrian - London, England