If the wild and native trout in the Streams of Yellowstone National Park see you, it will be next to impossible for you to catch them. In order to hide from them, you need to know a little about how a trout sees the world outside the water. The subject is far to complicated to discuss in detail in a short article but thank goodness, it isn't necessary. The above links to some previous articles gets into the trout's window of vision and how they see things both under and above the water. For purposes of what I want to point out in this article, just a little common sense and a little basic knowledge about how trout see is all you need to help you prevent them from seeing you.
Trout don't see objects above the water, especially those at a distance, clearly. To make it simple, lets just say they see things as a blur. They cannot see anything in detail at a distance. A person standing twenty feet from them is just a blurred image. Now for those who want to get picky, what I am about to say isn't exactly technically correct but I am not writing a scientific paper. I am describing how a trout sees someone trying to catch them. For all practical purposes, I am being accurate.
What trout will notice, much quicker than anything else, is the movement of an object above the water. They are used to seeing blurred images of objects above the water that remain fairly still. Trees and boulders don't move around a lot. Overhead predators pose a danger to them. Large birds and a animals pose a danger to trout. When something moves above the water, it gets their attention. The bottom line to this is that you should move as little as possible and when you do move, move as slowly as possible. Of course, it is difficult to cast without moving.