Tools of Characterization
Speech and Dialogue
This one really only works for Satan, but he's arguably the most important character so it's still important! Whereas God tends to speak in a sort of authoritative, stentorian (i.e., loud or powerful) voice, Satan tends to sound like a really passionate, persuasive rebel leader (just think of Mel Gibson in Braveheart or Russell Crowe in Gladiator, and then imagine they're evil). The thing about Satan is that he often uses loaded words like "tyranny" (as at 1.124) or "monarch" (1.42) to characterize God's regime in Heaven. In fact, he's a lot like that reasonably intelligent guy who's learned a lot of fancy words, and can get your attention, but really doesn't know anything. The force of words like "tyranny" and "monarch" repeated in the context of speeches about not repenting – always a favorite topic – gets us pumped up at the same time that it exposes the irony of Satan's situation: he's the real tyrant who has his own empire in Hell and is willing to destroy the lovely and innocent Adam and Eve just to spite God.
The other dead giveaway about Satan's speech is that it can often be very confusing; now, you might be saying to yourself, "Well, all of Milton is confusing." Good point, but Satan is easily the most confusing. Just look at his temptation of Eve. Yeah, sure it sounds straightforward but then he says things like: "look on me! / Me who have touched and tasted yet both live/ And life more perfect have attained than fate/ Meant me, by vent'ring higher than my Lot" (9.687-690). How can something be "more perfect"? When he speaks, it all sounds fine, but then when you stop and ask yourself what he's really talking about, you start to realize that it's more deceptive than you thought.
The way characters look matters a lot in Paradise Lost. Adam and Eve stand "erect" (4.288-289) (stand tall), an attribute that separates them from animals and implies a closer proximity to God. They're also naked – a fact that Milton mention several times – which is supposed to tell us that they live in a time where nakedness is not a bad thing. After the Fall, they put on clothes, and that is how God realizes that they've broken his one rule.
As far as heavenly creatures like angels are concerned, there is a very clear hierarchy. We know that Satan was once very close to God – like, really close – and his proximity to the God was signaled by the fact that he was much brighter than other angels. For example, we learn early on that in Heaven he "didst out-shine/ Myriads" (1.86-87) but he now he resembles his "sin and place of doom obscure and foul" (4.839-840). Similarly, when Raphael comes to meet Adam, the latter says that his angelic friend "seems another morn/ Risen on mid-noon" (5.309-310); in other words, Raphael is so bright that Adam perceives him as another sun rising in the middle of the day.
The same can be said of the various places Milton describes. Heaven is incredibly bright; nighttime there isn't really nighttime. It's really more like dusk. Hell, on the other end, is really dark; Milton says it has "No light but rather darkness visible" (1.63). It's a strange phrase, but Milton just wants us to know that his angels can see even though it's dark down there. The Garden of Eden too is an incredibly beautiful place; there are no extreme temperatures like in Hell (which has both extreme heat and extreme cold), and it is generally a very pleasant, innocent place, signaled by the plenitude of vegetation, fruit, water, etc.
While it is easy to be tricked by Satan and to get angry with God, Milton frequently steps in to tell us what we should really be thinking about these characters. For example, he says of Belial in Book 2: "He seemed/ For dignity composed and high exploit:/ But all was false and hollow" (2.110-12). Although he appears "composed" for "dignity," Belial is really full of it. We see something similar with regards to Milton's characterization of Satan. Thus, when he finishes speaking to Eve Milton writes: "He ended, and his words replete with guile/ Into her heart too easy entrance won" (9.733-734). Here we get a sort of double characterization: we learn that Satan is also a huge liar ("guile") and that Eve is rather weak when it comes to resisting Satan. Why else would his tricky words find "too easy entrance" into her heart? Moments like these are present throughout the poem.