Mark 9:6

τί ἀποκριθῇ, what he should answer to the vision; he did not know what else to make of it than that Moses and Elijah had come to stay. This is probably an apologetic remark added by the evangelist to the original narrative. Lk. reproduces it in a somewhat altered form. ἔκφοβοι : they were frightened out of their wits (again in Hebrews 12:21); explains the stupidity of Peter. The fear created by the sudden preternatural sight made him talk nonsense. Mt. makes the fear follow the Divine voice.