What happens after your fly hits the water is usually as important (or more so) than the cast you made to get it there in the first place.
If a fly behaves naturally, a fish will be more inclined to want to eat it than if it behaves unnaturally. So, for dry flies as an example, your fly lands on the water and it’s supposed to either replicate a dun which has emerged from its shuck, reached the surface and is drying its wings before taking off. Or your dry fly looks like a spent spinner, which has fallen back to the river surface and is about dead anyway.
In either case, that fly is vulnerable. Floating at the mercy of the current. It’s not supposed to look like a little sailboat ripping back and forth on a racecourse. That’s why you have to concern yourself with avoiding drag, when the current grabs your tippet and leader (or even fly line) and makes that fly move unnaturally.
Many times, I have been fishing, switching bugs, changing to lighter tippet, and so on, only to find out that what the fish really wanted to eat was a fly with a good drift. It’s amazing what a good angler can do with a Parachute Adams and 4X tippet when they know how to read currents, pinpoint their casts and mend the line effectively.
I honestly think I get more out of practicing and polishing mending than I do practicing casting. Of course, mending isn’t nearly as fun, and you can’t mend into a hula hoop in the backyard.
Streamer fishing is almost exactly the opposite, presentation-wise. Fish are not accustomed to their food attacking them. Streamers often simulate little baitfish. So, rather than pull your streamers toward the spot where you think the fish might be (or see it) drop that streamer in the zone and then make it try to escape.
Of course, there are exceptions to these rules of thumb. Dead drifting a streamer fly like a woolly bugger and making it seem like a leech can be pretty deadly. If you’re fishing streamers that look like baby salmon smolt, they’re drifting with the current usually with their heads facing upstream.
Some flies like caddis do skitter on the surface, and I know hoppers that fall off the banks into the river wiggle their legs. That’s why those rubber-legged terrestrial patterns work so well. I love skating dries at times.
I once fished a little creek in Chile, where we were throwing big Fat Albert flies to brown trout. I was in full stealth mode, quietly flipping casts against the undercut banks, trying to avoid any movement of the fly after it landed.
Nothing. Another few casts. More nothing.
Frustrated, I bombed a long shot right into the middle of the river and gave the line three sharp twitches.
Sure enough, a hefty brown charged off the bank and ate it with a thundering swoosh. The fish tracked that fly like a torpedo waking toward a target. I wasn’t sure if that was a Southern Hemisphere phenomenon, like the water in the toilet swirling the other way, or what, but I started doing that now and again once I got back home, and sometimes it worked.
I guess the real rule of thumb is that while you might endeavor to “spoon feed” with dry flies and “tease and aggravate” with streamers, sometimes you just need to mix things up to figure out what they really want.