In most SA revolvers, the hand can rotate the cylinder farther than is required to lock up. The cylinder stop will (normally) drop (yes, I know it goes up, but the term "drop" is used) into the cylinder notch before the had is at its full reach, so the cylinder is under pressure to rotate after it locks up. This puts excessive wear on the hand and cylinder stop, as well as on the cylinder notch and ratchet.
Ideally, though, three things should happen almost at the same time. The hand should turn the cylinder just to the locking point, the cylinder stop should drop into the notch, and the trigger should drop into the full cock notch on the hammer. When this is right, the revolver is said to be "timed" properly.
If your cylinder is rotating past the point where the cylinder stop should drop, the basic problem is in the cylinder stop, though the hand will also need shortening a bit. The cylinder stop should stop rotation whether the hand has reached its full travel or not.
When the hammer is cocked, the cylinder stop should first be drawn out of the notch it is in, then held down until it drops about 1/4" ahead of the next notch. Its spring should be strong enough to do the job, but most are far too heavy and will mar the cylinder. The cylinder stop part of the spring can be thinned to achieve the right tension.
It is possible that the cylinder stop is not coming up enough, that its spring is weak, or that it is not shaped right.
You can check some of these things with the cylinder out of the frame.
There are several books on tuning the SA revolvers, including one by Kuhnhausen, which is available from Brownells and contains everything you were afraid to ask about single actions.
Jim