When precisely in Paradise Lost does the "fall" take place? When Adam makes the mental decision to follow Eve's advice? As they actually share the apple? When their transgression is discovered by the angels? When they are expelled from Paradise?
As an overture, the question that first comes to mind is this: does Milton present any contrasting theological positions in Paradise Lost and De Doctrina? That is, is he writing for the same purpose, or does each work address similar subjects but for different purposes?
Similarly, might Milton's gloss on the supralapsarian and sublapsarianism doctrines (does Milton comment on the Synod of Dort?) inform our understanding of where/when the fall took place? Or, like Calvin, does he simply set forth the doctrine and then retreat from elaboration. Calvin is moving on for his own purposes, while Milton—or so it would seem—is using the question as a point of departure to display his learning, spin myth, and crack jokes to delight his fellow travelers?
The poem's initial descriptions of humanity's fall are not humorous, however. Consider Milton's descriptions of the universe going through various contortions.
When Eve plucks the apple from the tree:
Earth felt the wound, and Nature from her seat
Sighing through all her Works gave signs of woe,
That all was lost. (PL 9.782)
As Adam eats the apple:
Earth trembl'd from her entrails, as again
In pangs, and Nature gave a second groan,
Skie lowr'd, and muttering Thunder, som sad drops
Wept at compleating of the mortal Sin
Original; (PL 9.1000)
It is important to consider that in Paradise Lost it is not the contemplation of transgression that precipitates the fall, but the actual act of transgression itself. As evidence, consider Eve’s dreaming of disobedience in Book V. As Adam advises her, sinful thoughts are not themselves sinful unless they are “approved;” that is, acted upon. Too contemplate sin is not to sin.
Evil into the mind of God or Man
May come and go, so unapprov’d, and leave
No spot or blame behind: (PL 5.117)
So it would seem the specific act of eating the apple precipitates the fall.
Though the specific act of eating the apple precipitates the fall, the nature of that fall is rather curious. The conclusion of Book IX shows us that the "fall" is characterized by, first, enthusiastic love-making, and then, second, by an absurd domestic squabble, in which Adam and Eve blame each other for the transgression. Indeed, Milton's comedic hand is quite plainly evident at the end of Book IX: The "Fall" is an archetypical domestic argument between husband and wife. What is even more absurd is that in this case they are arguing over who is to blame for the Fall of Humanity. The passage is a delightful drawing-room farce, pure drollery.
Though, indeed, is there any state more "fallen" than a knock-down quarrel between a husband and wife?