Perhaps many of the posters are giving our quarry too much respect in relation to the size of the trout's brain! Granted if you are fishing small water where the width of the stream is less than 20' and the target is no more than 10' - 15' away from the caster then the fish may see you due to your movement and your line due to the fact that your leader may be in the 7' - 9' range.
I'd estimate that I catch well over a 100 17" - 22" fish every season and never give a second thought to whether I think my casting is inhibiting the trout from eating my fly. I seldom use a leader less than 12' long or heavier than 5x or 6x. Most of the time my tippet is 36" long. If the conditions warrant it I will go to 7x but in all but the most difficult fish 6x is very adequate. Possibly my success rate is based on the waters that I fish. I spend most of my time on either the WB of the Delaware or the main stem and two weeks in June on the Missouri. These rivers though are hardly pristine and underfished. The trout often see thousands of flies cast over them during the course of the season.
Use as long a leader as you can comfortably handle and a tippet no less than 30". I never cast directly upstream to a rising fish as I don't like the line and leader going over the fish. Ideally I prefer to be directly opposite a target or a foot, or two, below. Then I make my cast to land no more than two feet above the riseform. I take some guys out in my Hyde that I met on another forum and many of them throw way too far above the riseform than is necessary. What is the sense of throwing 4' - 6' above the riseform if the fish is most likely suspended within two feet of the surface? How do I know the depth of the fish? Well I don't exactly know the depth but if you see a trout rising steadily, every 5 - 10 seconds, there is a strong likelihood the fish is right near the top of the water column. The further above the target your fly lands gives it more chance to drag before it reaches the riseform.
On the Delaware I'm often in my boat and casting 35' - 50' to rising fish. At that distance they just are not going to see me, my false casting, (if you false cast away from the target)or my line. Actually if you are using a good leader why would they ever see the line? All they should be seeing is the leader and that might be all of .006" -.009" diameter. When I wade fish many of the rise forms I see are still 25' - 40' away and again I don't believe my casting or body plays into whether or not the fish will rise. If you feel the fish can see your form you can always crouch or kneel down into the water to keep as little of your form as possible visible to the trout's window.
On the Missouri there are certain lies that when the Tricos begin to fall the trout just clamor to get in the feeding lanes. The last few years there have been far fewer pods but there have been many times when I'd be floating by in my 'toon and see six heads coming out of the water at once. Fish that are feeding this agressively are far less likely to be even the least bit concerned with your casting motion. On pods like this I park the boat and cautiously wade upstream to get into position. On fish actively feeding on Tricos I like to get as close as possible so I see my tiny little bug. If the water has good depth I'll wade in to my waist and get opposite as mentioned before and then I pick out the bottom fish first and work my way up to the top fish so the others don't spook. If the water is shallow I still like to get close but it takes longer as I will get down onto my knees when I'm still 25' away and very slowly "crab walk" until I'm nearer the risers then the last few feet I'll knell down and just shuffle along the bottom until I feel I'm in the ideal casting range. (See Spinner "Smoke" below)
I don't want to oversimplify catching difficult fish because it is obviously not simple. Sometimes I may cast to a large trout for up to an hour before he/she eats the bug. All I am saying is that they may be cautious and wary but they aren't "smart" in our definition of the word.
Morning Spinner Fall