Could you please tell me the meaning of "casual" in the following excerpt from 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.
In this section of Chapter 2, Nick is both repulsed by and attracted (somewhat owing to being drunk) to Tom, Myrtle, and their friends. Nick recognizes a sense of elitism and thoughtlessness in these people. So, while he attempts to be friendly and accommodating, he also wishes to leave.
I wanted to get out and walk southward toward the park through the soft twilight, but each time I tried to go I became entangled in some wild, strident argument which pulled me back, as if with ropes, into my chair.
If he could be out on the street, he could be away from these people. Being outside and away from them also puts Nick in a more objective position. This is something Nick always tries to negotiate with himself: to avoid judging people, he must be sociable but also keep his distance. He must be "within and without."
Consider some definitions of "casual" such as: relaxed, unconcerned, occurring randomly, and informal. In this context in Chapter 2, Nick desires to be a casual watcher in the street. In fact, he imagines that he is the casual watcher. Here, "casual" means random, relaxed, relatively unconcerned. Nick is stuck in the apartment ("within") but imagines being ("without") outside of it, so he imagines being some random person on the street who might randomly gaze up at the yellow windows and wonder what is going on inside. Thus, as a random outsider, he would be free to imagine what might be going on inside and free from their company.