I could cast the fly a foot or two in front of the fish and strip it past and it might get up and chase the fly. Heck, with all the targets out here at times you could catch plenty of fish doing that and have a great time, but if you can put that fly on the dinner plate within a few inches without spooking the fish...it will flat out eat the fly. I have gotten pretty adept at casting close enough to get the eat, but not so close as to spook the fish. It takes a little feel, much like nymphing.
The issue is when to set the hook. If you do it right the tailing carp above will keep on happily doing exactly what it has been doing and your fly simply becomes another morsel to be sucked in and devoured...no wasted motion. Carp can suck in a fly from about a foot away without moving...I have seen them do it. With this in mind you can either guess (frankly, this works, if the fly is there and the fish isn't spooked it will eat that fly) or you can make em move. Instead of putting the fly inches directly in front of the fish, put the fly inches in front of the fish and 2-3 inches to the side of the fish. Make the fish turn its head ever so slightly and then key on the head turn, or on the tail pushing the fish around. You don't have to get the carp to chase, but you gotta make em move.