Charlie is pretty much terrified of new people, and even if he knows you or you're around a lot, it takes him a LOOOOONG time to warm up to you. But once he does, he's your buddy for life, even if you don't come around for months at a time, he'll remember you and come running when you come to visit

Other than that, they really aren't afraid of anything because if they start to display fearful behavior of a certain object, sound, or situation, I just ignore them. I don't tell them "no", I don't baby or coddle them, and I certainly don't tease them with whatever is scaring them. Eventually they'll learn to investigate whatever it is, and either learn to live with it or stay away from it lol