What you have to remember reading The Fall is that the fictional "you" is different than the you reading this book. The you reading this book is simply that: you. The one turning the pages. The one reading this website. The fictional "you," on the other hand, is a character in The Fall, created by Camus just as Jean-Baptiste is created.
What makes it even trickier is that the "you" character may be fictional even in the context of The Fall. In other words, it’s not just that Camus made "you" up, but Jean-Baptiste made you up. (Think of it this way: you’re reading a story about a guy named Jack. Jack is fictional, but in the world of the story, Jack is real. If Jack writes a story about a man named Ralph, Ralph is fictional even in the world of the story.) It’s unclear when you read The Fall whether the "you" character really is sitting at the bar with Jean-Baptiste, or whether the "you" character is a figment of Jean-Baptiste’s imagination.
One way to tackle this question is to look at the relationship between the "you" character and Jean-Baptiste. (We’re just going to refer to him as "you" from now on, but know that we’re referring to the man Jean-Baptiste is addressing.) Why does Jean-Baptiste want to talk to you? Why does he make you his "client"? Most importantly, what are you doing for him?
At the end of his story, Jean-Baptiste addresses most of these questions. And the answers are a little less than straightforward. In one sense, you’re the scapegoat, the one who gets judged. He throws all of these sins on you so he can condemn you as guilty and rise above you. In another sense, you’re supposed to take his freedom away so that he can be happily free of its burden. "After having solemnly paid my respects to freedom," he says, "I decided on the sly that it had to be handed over without delay to anyone who comes along" (6.17). And that anyone just happens to be you. You’re also there to help Jean-Baptiste break out of isolation. He says he "needs your understanding," because he’s given up on friendship and this is the best he’s going to get.
Perhaps most interestingly, Jean-Baptiste makes you into an accomplice. See for yourself: "I have no more friends; I have nothing but accomplices. To make up for this, their number has increased; they are the whole human race. And within the human race, you first of all. Whoever is at hand is always the first" (4.3).
So what does it mean for you to be an accomplice? What crimes are you helping to commit? As we know from other arguments in The Fall, "crime consists less in making others die than in not dying oneself" (5.17). Living in the first place makes you a criminal, so you and Jean-Baptiste are accomplices to each other’s crimes of being. But if you take to heart the theory that Camus’s novel is meant to condemn all of us, to illustrate, as the epigraph suggests, the vices shared by all of mankind, then you are an accomplice because you share in Jean-Baptiste’s sins: duplicity, hypocrisy, and self-serving egotism.