As Robbie said, it depends on many variables...
for example, a gin clear water would have the fish behaving in a different manner to a peaty water. The theory is that Trout do not have eyelids, so in bright conditions they will go deep..BUT..they are also oppurtunist feeders, so if there is a big hatch of buzzers, up they come. In a "dirty" water where light penetration is not so great, the fish may be holding higher up.
In rivers/burns, i would expect to find the fish under the cover of overhanging trees etc, but have also seen fish rising on really bright days.
I think the best thing is to just observe what is going on on the water, and also if other anglers are catching..and if they are, try and work out what they are doing.
For me, if i could not get any visual clues I would go with a very long leader, a heavy fly on the point and fish extremely slowly, letting the point fly sink deep, the middle fly will then fish higher up and the top fly "shallow". If i then get a fish, then I will adapt my tactics, either going onto an intermediate line if the fish took a deep fly or changing to a shorter leader with lighter flies if the fish took the top dropper fly.
Hope that makes sense,
rOB
