Cottonmouth at sunset (defensive posture).
This large snake was crossing the main entrance road to Everglades National Park right at sunset. I pulled over, got out and with my fully extended monopod I helped it across the rest of the way.
On the other side I moved it to behind my van where it was blocked from the view of the road. Then I set up the camera and took many shots.
Here you see what I would describe as a mild, initial warning posture. If the snake is really pressed it will widely gape its mouth, showing the cottony inner mouth and throw its head back. If pressed really hard a cottonmouth will actually hide its head under its body, apparently just hoping that the trouble will go away. I know because I have done a lot of experimenting with these snakes in the Everglades.
These snakes are not aggressive, but they will stand their ground when confronted. It is best to simply leave them alone, giving them wide berth.
Virtually all snake bites are caused by the person pressing the snake in some way, either by teasing it or attempting to catch it or even being bitten after it has been caught. These snakes do not attack.