By definition (at least mine) fishing is the act of an angler to search for the triggers to fish behavior of a population to bite. To choose to not recognize that these triggered responses may change within a population or as conditions do so with the water as a result (but not exclusive) of the season, water temperature, or daylight levels through out the day is just not productive.
You may have to realize at some point during a day of angling.......that you simply chose incorrectly. It's ok to be wrong....
The fly does and doesn’t matter. A fly may be too large, or too bright or too flashy. We’re looking to trigger the predatory cat chasing the mouse instinct in a fish. There are negative attributes a fly can have that will inhibit this trigger. It’s up to the angler to figure these out through a method using trial and error, experience and feedback. Watching a buddy or other anglers success or lack there of points you to the direction of what works....assuming of course that there are fish and they are willing participants that day.
In general, will a cat chase a baby elephant that’s drug across the floor? Or equally so, will it chase a grain of sand in the same method? Likely not. Will it chase a 2" sized fluffy nugget that's drug across them and does it matter if it's black or white or clown colored? It depends......but more times than not.......it doesn't matter if they are a willing participant to the game. Throw in environmental changes like light levels, rain, stress that alters their behavior....... well you're trying to crack the code now aren't ya? We can think of all of this in human terms....but sometimes you have to think like that fish. Sometimes we outfox ourselves.
What am I saying? I pick a fly I think will swim right, is appropriate sized, looks good to me and present it in a manner I think they will respond. If that doesn't work after some time or I see others around me having success and I am not...I will change. To what? That's where you're own brain will have to come into play. Because some things matter and some things won't. Have a planned systemic process where you cycle through colors, flash, speed, or water column depth,etc. There is no one process and no two fish behave the same......you're trying to appeal to the majority of the minority that will even chase your damn fly.
The above is all purely speculation and my theory to how the fish we target respond to the fly we present to them. Try to think of it in statistical terms. If you plot all flies that have caught fish ever on a chart and choose the single fly that's caught the most (a black leech let's theorize)......you have a pretty good chance of success under any condition. Now.....if you look in your box and just close your eyes and grab whatever......are you going to have equal success? If you're Adrian, yes....for us mere mortals....I'd theorize no. Some might say they'd could out fish you with a bare hook...I'm still looking to see that individual. Must be Bigfoot's brother.
Am I right? I've caught some fish....so I'm not 100% wrong is all I can say. Perhaps all that I do, and believe is pure and utter nonsense and is a blind squirrel finding a nut. I'd like to think not....... but at least I have a process I follow that gives me confidence with a given choice. Every cast and swing made....I think is the one. 99.99% of the time I've been proven wrong....so what the hell do I know?