And he and all they that followed him were cast out
So, when Lucifer was cast down with all his devils, where did the others go, when he came and tempted Adam and Eve? Consider this verse from Moses 4:6. For clarification, the brackets denote my input, and the parentheses are an actual part of the scripture verse therein.
6 And Satan put it [his evil spirits] into the heart of the serpent, (for he had drawn away many after him,) and he sought also to beguile Eve, for he knew not the mind of God, wherefore he sought to destroy the world.
Remember, if Satan can take over the heart (and the serpent was already the best candidate for this) see Moses 4:5 below, than he can control the whole creature.
5 And now the serpent was more subtle than any beast of the field which I, the Lord God, had made.
So, with the evil spirits dwelling in the heart of the serpent, Satan could then command it, as he does his followers, and with them, he spoke to Eve, through the body of the serpent.