when the fish are active up in the surface area, and especially if you can see insects being blown on to the water surface,and being taken by the trout, it is worth using a dry fly. before the fly is cast it should be dunked in a bottle of floatant.
Dragging the fly accross the surface in short erratic jerks will often produce a response.
A floating line can also be used to fish a nymph on or very near the bottom. In this case the leader will have to be longer than usual, (15ft) and the nymph will need a weighted body.
Whenever you are using nymphs always remember to wipe the leader with "leader sink"
Sometimes a trout will be seen leaving a trail of rings as it cruises just beneath the surface, sucking down insects that lie in its path. by logical deduction it is possible to present your fly accuranty ahead of the fish.
as nymphs or pupae rize to the surface to hatch, they are often intercepted by the fish before they reach the surface. This activity is perceptible only to the keenest eye. Binoculars are a great help when trout feed like this.
After the cast, pause a while to let your nymph sink well beneath the surface, then retrieve the line slowly, pausing occasionally to keep the nymph about 30cm under the surface film.