The main difference between Paradise Lost and the account of the same events in the Bible is that Milton's version is much longer and more detailed. The story of Paradise Lost covers only the first three chapters of the Book of Genesis, a matter of eighty brief verses, a small fraction of the length of Milton's epic.

This difference in length means that Milton has added a great deal to the biblical narrative. One of the most important differences is his choice to begin the poem in hell and to focus throughout on the figure of Satan. Both hell and Satan are much more shadowy presences in the Bible, and the reader certainly never hears the viewpoints of Satan and his lieutenants there. The serpent in Book 3 of Genesis is not even explicitly identified with Satan, but is merely "more crafty than any other wild animal." Similarly, the battle scenes with modern weaponry and Satan's role as inventor of the cannon are Milton's additions.

Another important change in Paradise Lost is the addition of long, eloquent, dramatic speeches, particularly between Adam and Eve and between God and the Son, as well as the rousing martial words of Satan. These add drama and psychological subtlety to the events depicted, allowing Milton to create more complex characters and to develop his own theological viewpoint in describing their interactions.