They can only do that for a time however, eventually they either have to become desensitised to most things or else you have to wonder why the hell they're travelling with the Doctor? (you can't pull the old "I can't get you home" trick ala Ian/Barbara or Tegan these days given the greater control the Doctor seems to have over the Tardis.)
Amy was definitely scared shitless in the early days on occasion (particularly where Weeping Angels were concerned.)
Amy was definitely scared shitless in the early days on occasion (particularly where Weeping Angels were concerned.)