Sartre describes a café waiter whose movements are just a little too precise, whose attention just a little too eager, whose tone just a little too servile. The waiter is playing at being a waiter — performing a role so completely that he avoids confronting his own freedom. He is trying to be a waiter the way a stone is a stone: necessarily, without choice.
Bad faith is a form of self-deception peculiar to beings who are free. A stone cannot be in bad faith; it has no freedom to deny. Only beings who know at some level that they are free can flee from that knowledge. The flight itself betrays what it denies.
Bad faith is not merely hypocrisy — lying to others. It is lying to oneself: persuading oneself that one has no choice, that role, nature, circumstance, or God determine what one is. The authentic life, for Sartre, is the life that faces its freedom squarely.
Sartre himself was accused of bad faith on many occasions — most notably by Simone de Beauvoir, who argued that his political positions involved exactly the kind of denial of complicity he criticized in others.