Sight fishing, and fish watching period, is really fun. I’ve learned so much from it. Each fish, and set of conditions, is different. But there are some patterns.
-The first line of defense is the startle reflex. If you are calm and fluid, and patient, you may not trip that wire. Fish, and other critters, can get used to your presence, eventually filtering out the startle reflex -called "stimulus extinction".
-Lighting and visibility conditions affect things, with bright sun making fish edgier, spookier, and making it MUCH easier for them to see, esp details.
-Each fish is an individual and some are simply more cautious than others.
Deciding on what to do when you spot a fish is the tricky part.
I suppose the thing you DON’T want to do the majority of the time is freak out and make quick movements. I also have a habit, from years of hunting, of averting my eyes, looking obliquely at critters –not staring. With fish this isn’t such an issue, they don’t understand the stare, but they can pick up on the movements you make in getting into a stare posture, quickly bending down toward them to duck glare or concentrate your vision. If you’ve ever shot video of your friends hunting or fishing, it can be a big surprise to them how they ACTUALLY look when reacting to something of great interest, like a sighted bass. Often they’ll wiggle all over or just make subtle adjustments that alarm critters. This is sometimes why people who aren’t trying to sight fish catch them, by ignoring the fish until it ignores them, then casting. It’s a tack I use in both fishing and hunting. Although fish do get jaded toward fishing, more cautious in scrutinizing baits, I often wonder how much is due to their perceptions of our presence. It certainly can matter a lot.
Another option is to spot a fish, mark it, and return in 1 to 10 minutes depending on the circumstances. Bass are not often stationary so you often have to cast immediately. Careful motions, casting to one-side and ahead, or “tower casting” way high above and beyond sometimes works. Sometimes dropping it right on them works, although if it’s bright and clear and/or the fish is aware of me or even mildly startled, I’ll almost always cast out and retrieve to the fish.
Fun stuff.