How do Milton's interpretation of Genesis differ in Paradise Lost?
Milton's avowed aim in Paradise Lost is to justify the ways of God to man. His emphasis, therefore, is somewhat different from that of Genesis. Milton is a humanist, albeit a Christian one, so his main focus throughout is on the tragic consequences of man's first disobedience.
Milton's humanism also leads him to include the fall of Satan as an important element in the poem. This is Milton's own addition to the creation story of Genesis. He needs to give Satan such a significant role in man's downfall in order to show the limitations of the human mind. Milton's overall purpose in writing Paradise Lost is didactic; he is making an attempt at nothing less than the inculcation of virtue in his readers.
The Genesis story, on the other hand, purports to provide an account of how the universe came into being. It is cosmological where Paradise Lost is moral. It sets out a God's-eye perspective on creation rather than a human one.
In keeping with Milton's humanism is his insistence on free will in human nature. This is a gift from God which Adam and Eve can choose to exercise for good or ill. Satan can tempt us, but temptation must be resisted. And that requires the use of our rational free will. Independent reasoning can go one of two ways for Milton: either it can lead us closer to God; or, as with Satan and Adam and Eve, it can lead us completely astray. Human reason is not the problem; it is how it is exercised that matters, and to what purpose.
It is also of note that Milton changes the Genesis account of creation by having Jesus the Son, rather than God the Father, create the earth and the heavens. There are two possible reasons for this. One is Milton's unorthodox embrace of Arianism, the heretical doctrine that Jesus was somehow less than God within the Holy Trinity. Another is that Milton uses the figure of Christ as a bridge between the utterly transcendent God and the equally imminent human figures of Adam and Eve. Whatever the reason, Milton's fresh take on the creation story represents a radical departure from Genesis.
In Paradise Lost Milton is by no means trying to rewrite Genesis. However, what he is trying to do is flesh out the original text in order to give us an object lesson in virtue and the correct way for a Christian to use his or her God-given capacity to reason. Milton grabs cripture with both hands, as it were, and brings it down to earth. In doing so, he seeks to bring God and man closer together, while at the same time doing justice to their respective characters.
One of the major aspects of Milton's account of the Book of Genesis which is different is the way that Milton focuses on the freedom that God gives to man. God creates them "free to fall," and this is something that is shown to be essential to their very nature. They must be able to defy God if their relationship with him is to mean anything at all. This is why in Book III God says that he "formed them free, and free they must remain." Milton therefore majors much more heavily on the whole aspect of free will.
This is because Milton believed that choosing to be obedient was to be seen as a sign of mankind's adoration to God. Obedience has to be a choice for it to mean anything. If it were a pre-programmed, robotic response, it would be meaningless. A key theme of Milton's rewriting of Genesis is therefore this aspect of free will and how God created Adam and Eve to possess it, even though this opens up the possibility of their disobedience.
Apart from this, there are many other differences such as the role of Satan and the background we are given about him. Milton parallels the Fall of Man with the fall of Satan and his minions, and the two provide a very interesting counterpoint.