‘And so fearful was the appearance, that Moses said, I exceedingly fear and quake.'
But the experience was so dramatic that even Moses found it hard to bear. We tend to forget that Moses was human too, and that he was dealing with something that was beyond his understanding. Compare Deuteronomy 9:19 LXX ‘and I was greatly terrified because of the wrath and the anger' in respect of the golden calf experience and Exodus 3:6, ‘he was afraid to gaze on God', in respect of the burning bush experience. Moses trembled there before God, and here too he trembled along with all the people (Exodus 19:16). The writer puts words into Moses' mouth, based on what is revealed about his experience so as to make it more vivid (note that he does not represent it as from Scripture), probably based on some well known Jewish tradition. Such tradition often mentions Moses' terror.