I would look at a guy who started fishing right next to me on a stream like I would look at a guy who sat right next to me in an empty movie theater. Or a fellar who climbed a tree right next to me in the deer woods. "Are you serious, buddy?" I'm not anti-social in the woods, but I would just see that type of crowding as very strange, and frankly, rude.
On the other end, a quick hello, a short exchange, the usual "How they biting?" or "Any on the surface?" is generally acceptable and polite conversation before walking far enough away that you are at least out of sight on a mountain stream.
That being said, I have never experienced anything remotely rude or out of the ordinary from another fisherman in the park. Kayakers are another story.