Mark 9:4

there appeared unto them The three Apostles had not witnessed the beginning of this marvellous change. They had been weighed down with sleep(Luke 9:32), lying wrapped like all Orientals in their abbason the ground, but awakened probably by the supernatural light, they thoroughly roused themselves(Luke 9:32), and saw His glory, and the two men standing with Him. It was clearly no waking vision or dream.

Elias with Moses (i) Among all the prophets and saints of the Old Testament these were the two, of whom one had not died (2 Kings 2:11), and the other had no sooner tasted of death than his body was withdrawn from under the dominion of death and of him that had the power of death (Deuteronomy 34:6; Judges 9). Both, therefore, came from the grave, but from the grave conquered. (ii) Again, these two were the acknowledged heads and representatives, the one of the Law, the other of the Prophets (comp. Matthew 7:12).

they were talking St Luke tells us what was the subject of mysterious converse which the Three were privileged to hear "the decease, which He was about to accomplish at Jerusalem" (Luke 9:31). St Peter himself reproduces this remarkable word in his second Epistle Mark 1:15. "Vocabulum valde grave, quo continetur Passio, Crux, Mors, Resurrectio, Ascensio." Bengel.