The Woolly Bugger and its mate, the Woolly Worm, would probably be the most common flies used in fast, deep runs. They are generalist patterns, designed to attract fish from their movement, rather than being an exact interpretation of a specific trout food. So they have to be fished with a technique that gives them some movement that will catch the attention of the fish. In cold weather, any wet fly or weighted nymph should be fished dead-drift close to the bottom, as fish will only move a few centimetres to intercept food. They are conserving energy, as they would not be energy-efficient if they were swimming around chasing food when they are using a lot of energy to fight the cold. So the best technique is to cast across and slightly upstream. This will give the sinking line time enough to drag the unweighted fly to the bottom where the trout will be lying to gain shelter from the heavy current. If you happen to be in a part of the river where spinning is permitted, then you can use a weighted wet fly to get down quicker. But in fly only water where flyfishers prefer to fish, you will have to give the line time to get your fly down to the bottom of the run.
While the line is sinking, it is necessary to prevent slack line from forming a bow between the rod and the fly. This bow will be seized by the current and drag your fly downstream faster than is natural. That might fool a juvenile fish but an experienced trout will recognise there is something awry here and have nothing to do with this unnaturally-moving item. As the line sinks in the water and passes you, then the rod can be lowered to lengthen the drag-free drift. Additional line can be fed out to further lengthen the drift.
A few twitches or wriggles of the line can be tried at this point to catch a trout’s attention. As the fly reaches the bottom point of its traverse, the fly should be allowed to swing round and start to rise as the taut line pulls up the fly. Imparting a few movements to the fly at this point is often very productive. As the fly drifts in towards the bank, a few twist retrieves may then be worthwhile to check that there is not a big brown lying quietly in the shallows. A roll cast will then clear the line from the clutch of the water and a single false cast will have the fly back in the river. No matter how good an angler you are, if the other guy has his fly in the water 50% more of the time, he is going to catch more fish. Just remember that when you roll cast, you should make sure the line lands on ‘dead’ water and not on water where the fish may be lying. And hopefully, when you do land that big brown using the above technique, you will return it to the river, even if it is not from a mandatory catch and release zone.