The Fall has been characterized as Modernist because it is so steeped in uncertainty and because Jean-Baptiste seems to defy any clear interpretation. Check out this passage:
Isn’t it the most beautiful negative landscape? Just see on the left that pile of ashes they call a dune here, the gray dike on the right, the livid beach at our feet, and in front of us, the sea the color of a weak lye-solution with the vast sky reflecting the colorless waters. A soggy hell, indeed! Everything horizontal, no relief; space is colorless, and life dead. Is it not universal obliteration, everlasting nothingness made visible? No human beings, above all, no human beings! You and I alone facing the planet at last deserted! The sky is alive? You are right, cher ami. It thickens, becomes concave, opens up air shafts, and doses cloudy doors. Those are the doves. Haven’t you noticed that the sky of Holland is filled with millions of doves […]? (4.2)
This, Jean-Baptiste’s description of Amsterdam, could just as well be an account of his narrative, as many, many scholars have pointed out. It’s a "negative landscape." "Life is dead." You and Jean-Baptiste are alone, facing a dismal and "deserted" "planet." The dove business even borders on the surreal, which reminds us that we are in a world that doesn't quite make sense, and there is a foggy uncertainty to this text in general. All of these elements are calling cards of Modernism. Works in this genre tend to emphasize the limitations of reason and the inherent "unknowability" of the world and everything in it. Clearly, this is not your traditional novel.
Now we can talk about philosophy in The Fall. Camus was a fan of philosophical literature; he would use fiction to expound his theories. This doesn’t mean that Jean-Baptiste is arguing on behalf of Camus; sometimes the fictional characters serve as negative examples. Sometimes the lessons are ironic. Needless to say, it's a bit tricky to nail down the author’s point of view. Still, we can identify in The Fall some classic Camus ideas.