To successfully fool a fish into striking, one must use the right gear and present the fly in a natural manner. One of the biggest aspects of using the right gear involves choosing a fly that looks realistic. Sure, sometimes fish may react to a not-so-realistic pattern out of aggression and anger, but in other instances you've gotta show them something that looks legit.
Much of my fly tying involves tying relatively simple streamers ranging from Woolly Buggers to general baitfish patterns. When constructing these flies (and of course, others), I'm always very picky about what goes into the fly. Weight, eye color, fiber colors, the amount of flash, and any accents are always taken into careful consideration. One of the first "stop and think" moments when actually tying many streamer flies involves measuring the lengths of fibers that will be used. In a nutshell, it's often important to get it right the first time.
With some patterns such as synthetic baitfish streamers, you don't have to worry about being too exact because these unnatural fibers can or should be trimmed to shape after tying is complete. However, with flies utilizing natural materials such as bucktail or marabou, I want to make sure to tie-in the fibers at just the right length so I don't have to trim anything later. What's the big deal with getting these fiber lengths measured properly?
Natural fibers (like the bucktail or marabou mentioned) will have a slight taper, meaning the fibers are thicker at the base but the tips will be thin and pointy or wispy. When these fibers are tied-down, this taper adds to the overall realism and the fiber ends will also be slightly irregular with a nice, natural appearance and profile. These thinner tips will also help produce a touch more action. On the other hand, if care is not taken and these same fibers are tied-in at the wrong length and given a straight cut later to shorten them, now the tips will be removed and they'll all end in an abrupt straight line. This gives the fly more of an artificial appearance and can also cut down on the action somewhat. It's your call, but I generally avoid trimming these natural fibers at all costs when constructing something like a tail, wing, or body. Can these trimmed flies still catch fish? Sure, but for me in most scenarios, if a fly doesn't look as natural and doesn't produce the optimal action, personally I'd rather fish something else.
Tie your flies carefully and avoid an unnecessary hair cut. Your flies won't only look better, but I think it'll help your success, too!