A rattlesnake's first reaction upon approach is immobility. The snake will usually lie perfectly still in hopes that the camouflage will do its job.
If molested, the snake will usually coil and rattle. The loose skin under the throat is expanded in a warning gesture. The snake usually puffs its body up in an attempt to make it appear larger. The snake will use a combination of thermal tracking with the pits, and smell by tongue flickering in order to obtain information about its perceived threat.
If given a chance to escape, the snake will usually back away cautiously.
Click on FRED the albino eastern diamondback rattlesnake for the next page in the site tour. :-)