In cool weather, the snakes will crawl onto rocks or logs to bask. Snakes will also shade themselves in the overhang of a rock or log. It behooves hikers to watch where they put their hands and feet. Venom and the hypodermic solenoglyphous fangs are the first adaptations to come to mind when considering rattlers. Pit vipers have a very sophisticated venom delivery system.
Adults have full control over the amount of venom injected and through which fang, one or both.
Many startle bites are indeed dry bites with no envenomation. This fact is probably behind the steadfast belief in the efficacy of folk treatments for rattler bites. The facial pits are a very obvious feature in pit vipers like rattlers. The pits, behind and below the nostrils in front of the eyes, are larger than the nostrils. These passages are reproduced in the Info Page on Reptile Physiology.