The Great Gatsby, casual watcher

Could you please tell me the meaning of "casual" in the following excerpt from The chapter Two of The Great Gatsby?

Yet high over the city our line of yellow windows must have contributed their share of human secrecy to the casual watcher in the darkening streets, and I was him too, looking up and wondering. I was within and without, simultaneously enchanted and repelled by the inexhaustible variety of life.

Thank you.