Tools of Characterization
This one is easy. The sphere you occupy reveals a lot about your character. We know everyone in Heaven is virtuous and good and almost perfect, so what we're talking about here is distinguishing the minute differences in perfection.
If you appear in Mercury, you were good but all you cared about in life was fame. Live in Jupiter? Congratulations. You were probably royalty and a just ruler. Saturn? You devoted your life to religious meditation. Flitting around the Primum Mobile? You became an angel. We think you get the idea.
We put this in quotes because even though the souls think of their emanating light as clothing, that's not really accurate. Basically, the brighter you are, the more virtuous you are. Piccarda was about as bright as a pearl so she's on the low end of the perfection scale. St. John, on the other hand, is so bright that Dante goes blind looking at him. Christ is as bright as the sun and Mary is a living star. And God? You can't even look at Him. You need permission from Mary first.
If you're red like St. Peter, you have a grudge against the corrupt popes. The angels have a bit more regalia to their raiment; they've got golden wings. And finally, what does Dante wear? His body. He doesn't shine or dazzle. He goes blind once in a while too.
Okay, so all of the blessed have pretty much the same habits—smiling, singing, dancing, praising God. Let's look at the habits of the blessed while back on Earth. Those who broke their vows even though they didn't mean to can still make it to Heaven, but they'll be put in the lowest sphere. Those who fought in the Crusades are virtuous, even though they killed people. If one meditated a lot, he's probably up with the contemplatives in a relatively high heaven.
See what we mean? They're all basically good.