They can surprise you. My brother and I were leaving a swampy area one day and the very last step coming out of the swamp a cottonmouth lunged at his (my brothers) heel. It missed thankfully. It was under a log we stepped over to get out. Then on the other hand a friend of mine and I were working on a fence on some of his land one day. We were crawling under a log instead of climbing the fence. We had crawled under this log 10 times or better when all of a sudden a very large copperhead came crawling out from underneath. My stomach churned lol but it didn't bother us and I guess we didn't bother it to much either lol.