The better you get at seeking out and finding fish by sight, the more experience you have with the different actions and reactions of your favored species. As you approach a fish and then work hard to sight cast and catch that fish, you will begin to notice its little changes in body postures and actions that will allow you to "read" the fish -- to get to know it so well that you can anticipate its next moves.
Over time, you'll develop a kind of sixth sense of when you need to back off, be more aggressive or make another cast. You'll know the course of action you need to take to get the fish to eat the bait whether it wants to or not. That knowledge of a variety of species, along with adept use of a variety of casting, spinning and flyfishing tackle, is a mark of the accomplished angler.
Each fish species displays consistent reactions to threats and excitement, and the better you focus on watching fish closely and then reacting to their postures, the better you'll get at sight fishing. Sailfish, for instance, will light up and change colors when excited or feeding. When you see a fish light up, you know it's having a positive reaction to your offering.
Over time, you'll see that fish strike a lot of different postures that, if read properly, will help clue you in to their mood. Let's say you're sight casting to trout in a stream and watching a fish as it reacts to stoneflies landing on the water to lay eggs. As the big flies are swept into the fish's field of vision, it moves to a point downstream where it can easily intercept a meal.
When you throw your fly upstream of that fish, it moves into position, but when the fly gets close it doesn't eat. You know the fly has evoked a positive reaction from that fish, but something is keeping it from taking the bug as it gets closer. Likely there is something wrong or weird about the fly, or the fish is seeing the leader. It might mean the fly is tangled, the coloration is a bit off from the natural stoneflies or that the size is wrong. Or it can mean the tippet is too heavy and the fish is seeing it just before it eats. Or the fly is being pulled unnaturally against the current. Watching the posture of the fish over several casts will give you an idea of what is taking place and a simple change will get the fish to bite.
There are times when you'll see fish change postures from a comfortable position to one on its guard. If that fish hasn't moved off, it likely detects your presence but isn't sure where you are. It still won't eat until it relaxes.
In this instance you want to back off the fish and change the angle of approach. Watch the fish closely from a distance and wait for it to take on a more relaxed posture or a feeding posture before making the longest cast possible. Often, these changes will elicit the strike.
Off a saltwater beach, spawning-size jack crevalle swimming in a counter-clockwise circle will reveal their mood by moving up or down in the water column and finning, flashing and generally swimming steadily in a circle on the surface. When those fish feel threatened, they'll drop a bit in the water column where their fins don't protrude from the water, or will break off from the circle and move as a line away from the approaching angler. In this case, you want to back off the school and wait for it to calm down, circle up and start swimming on the surface again before making a cast.
Many times, the posture of a fish will let you know when you are about to get a bite. Tarpon are thrilling to watch as the fish flare their pectoral fins and hump their bodies when pursuing a fly or bait. When a tarpon "humps up," you pretty much know the bite is imminent. The classic quiver of a bonefish tail as it accelerates to take a fly also comes to mind.
The thing you want to remember about fish postures is that they are one more indicator as to what the fish might be experiencing and how it is going to react to your offering. If you see a negative posture, with the fish tensing its body, twisting away or even with no change in its posture, it likely isn't going to chase your bait. When you see that positive posture like a spreading of the pectoral fins (everything from sailfish to redfish to brook trout do this), the arching of its body, turning rapidly toward the bait or even simply going from still to a slow but steady swim in the direction of your offering, you can keep your eyes on that reaction all the way until the fish strikes. That anticipation of the fish's action and the confirmation of it by sight adds to the sport's enjoyment.
Watching Actions and Reactions
Fish are finicky, and what works one day may not be what they want the next day, or even the next cast. Watch the fish closely for those reactions that tell you a fish is interested, and then duplicate or improve on that action. If the fish are disinterested or turn away, you know to try something different to get the bite.
Cobia are a great example. Casting and watching their reactions to initial presentations often leads to a solid hookup. There are days when you'll encounter surface-swimming cobia that eat everything you throw their way. Then there are those days when they seem like the pickiest fish in the ocean.
Cobia love the color chartreuse, so much so that a large percentage of anglers who sight cast cobia leave the inlet with at least one rod rigged with a chartreuse jig or soft-plastic swimbait. But I've seen times when cobia will not even react to a well-cast chartreuse jig, and then changed to a chartreuse swimbait, and still had no reaction. I've then changed to a chartreuse surface plug with the same results. Then someone on the boat throws an orange jig and the fish climb over the backs of each other to get to the lure.
Direction of Movement
Any time you're trying to sight cast to fish, you want to determine the direction of their movement or the direction the fish will move to feed. Spotted sea trout commonly lunge at and grasp their prey, usually traveling no more than a few feet to catch it. A spotted seatrout sitting stationary will feed in the direction it is facing. That means it is waiting for food to approach from a specific direction. Your lure, bait or fly needs to pass within one or two feet and directly in front of that fish.
The most commonly encountered stationary feeding fish are facing into a current and waiting for their food to be washed into their strike zone. That doesn't mean the fish is going to eat the bait as it approaches. Snook sitting under the lights of a bridge or dock at night will utilize the light to locate shrimp or fish, but will regularly wait until the bait has passed before spinning around and eating it. Usually that bite takes place a few feet downcurrent of where the snook is positioned under the light.
A moving fish can be easy to target because you know the path it is going to follow. Lead the fish by casting ahead and into its path and then wait for the fish to approach the bait. With a lure or fly, you can rest that offering until the fish moves close enough so that the bait is in its field of vision and then start the retrieve. That action gives the fish the impression that it swam up on a bait which tried to escape, drawing a reaction bite.