The devil had taken the form of a serpent or somehow possessed and spoke through a serpent so that it can accurately be said somehow that he was the serpent. The Book of Revelation makes this clear:
Revelation 20:2 "And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years"
Revelation 12:7-9: "And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, And prevailed not; neither was their place found any more in heaven. And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him."
He was involved in the fall of man by beguiling Eve into believing that eating from the tree which God said not to eat from was actually good for her, would make her as a god, and not cause her to die like God had warned. He also knew that this was a potential way to get Adam to transgress through his attachment and sympathy to Eve. Satan speaks through his servants today also to tell us we can have life in Christ while we also disregard God's authority and violate His commandments- and hence those who heed his doctrines which imply this (though they don't usually come right out and say it) are kept in the death of their fallen state and vaccinated against the gospel with a false hope of heaven.
2 Corinthians 11:2-4: "2 For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ.
3 But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ. 4 For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him."
