The Element of Choice

I think one of the major themes of both Paradise Lost and Good Omens is that of choice. The most critical events of both these works have to do with a character making a conscious decision about whether or not to take a certain action.
In Paradise Lost, Satan makes the choice to rebel against God, taking his followers with him on a course that leads to expulsion from Heaven and damnation to Hell. Among those followers is the angel Abdiel, who chooses to repent of his errant ways and return to the folds of Heaven. Later, Eve makes the choice to obey the serpent and eat of the Tree of Knowledge, thereby willingly disobeying God. And when she goes to Adam and tells him what she did, he makes the choice to follow her in sin and also eats of the Forbidden Fruit.
In Good Omens, the whole idea of choices defining who we are is revisited. There are several instances where Crowley and Aziraphale go against what they believe might be expected from them by their superiors in order to carry out what they see fit to be the right order of things. For example, Aziraphale chooses to give the Flaming Sword to Adam and Eve because he takes pity on them, even though he's supposed to be using the sword to guard the gates of Eden. Much later in the book, Crowley actually defies the demons sent to capture him (including the Duke of Hell) in order to go find Aziraphale and see what's actually happening. Perhaps the most striking example of choice in Good Omens is the one involving the boy named Adam, who's namesake was part of the choice which brought about the Original Fall. Instead of doing evil deeds and undertaking hellish endeavours, Adam chooses to do things that he and his friends believe to be the nice thing for the world, like growing rainforests and freeing the wales. More importantly, Adam makes the conscious choice not to destroy the world with his awesome powers.
In both of these works, choices are the crux on which are formed the main events of the stories. All three of the authors want to stress the importance of the fact that our choices define us, for better or worse, not who we were born or the name that is given us.