I've had some great conversations, with intersting people. But the best was when I was seated next to the wife of the Captain, it was his last flight before retirement, and his wife and daughter were invited on board to share this with him. As I was sitting next to her and had chatted during the flight I was included in the celebrations. It was very emotional and great fun.
I'll normally nod and say hello to my neighbor, but if conversation is not wanted or is difficult I just open my book an get lost in that.

Stewart