You had better believe this. You will never see it until he moves and then it may be too late.
The first one I killed at my place. Holding a running pole saw, looking up at what I needed to cut next. Looked down, stepped forward, then realized I had just stepped over a copperhead. He didn't move. 230 gr JHP to the head.
Second one I hit on the tail with a weed eater, and he struck at the line, then tried to go the other way. 230 gr JHP to the head.
So, I've not been attacked by them. But cottonmouths, they are dickheads. They will chase you.
Be armed, always. 4 legs, 2 legs, no legs. There's always something that needs killing.