I usually just try crap out when I find people that aren't trying to kill me. Found out early on that people and friendly creatures were interactive when I walked up to one of the Combine guards. I pressed e, he said "move along", did it again and he lit up his batton. Just for the hell of it, I did it again and he hit my ass. Screen went white and I woke up about 40 feet away. I tried it again, but this time stood a bit back and began to ran away when he moved. He chased for a while, then stopped and went back to his post.
-SW