Joe Brown and Joe Christmas are foils. Not only do they live in the same cabin near Joanna's house, but Faulkner also invites the reader to compare them by giving them the same first name. The name is fitting for both of them: simple and common, it helps them blend in as they drift from place to place and try to hide from their past. They each operate a bit outside the law and outside the strictures of social respectability. Christmas has killed his adoptive father and has an illicit affair with Joanna. Brown has abandoned Lena and is making money off of an illegal bootlegging operation. They both know each other's secrets and live together not happily, exactly, but with the understanding that betraying one another would bring mutually assured destruction.
Until Joanna's murder, it almost seems that Brown is the one of them more likely to be run out of town. After all, he does not try very hard to conceal his bootlegging. He causes drunken trouble about town, whereas Christmas keeps to himself. At first, because of his reputation, Brown seems like the likely suspect for Joanna's murder. However, there is a key difference between the two men that saves Brown and condemns Christmas: Christmas is rumored to have Black ancestry. Even though no one can ever confirm whether or not this is true, the simple possibility that Christmas might be Black instantly saves Brown from suspicion. He is able to get away with a litany of dubious behaviors simply because his Whiteness protects him. Christmas has not been entirely without the protection of Whiteness. After all, until now he has been passing for White in Jefferson. The townspeople have treated him as a "foreigner" and have mostly left him alone. Were he more obviously Black, he would not have been offered the same freedom. Still, the instant his possible Blackness is revealed, the entire town turns on him. The doubling of these two characters in one another demonstrates that in the world of the novel, racial identity determines everything about how people perceive one another.