Most fly fishers take river etiquette really seriously. I want to consider them like knights of medieval times with their polite codes of honor. They took pride in just what they were doing and also in the reputation they were supporting. The exact same opts for fly fishers- appreciating the river and those on the stream is a large offer.
A good rule of thumb is to begin out assuming that every fly fisher on the river is your close friend (you never recognize, someday they may be), and treat them appropriately. You would not shout at your friend, or try to swipe their waters.