In HP, we have four categories of characters. A, B, C, and D
These are Harry, Hermione, Ron, Hagrid, Snape, Malfoy, Dumbledore, Voldemort, and Hogwarts.
Each of these characters get a unique take. They are each used differently. Do not expect death lightly here. These characters provide the tension of the book. Do expect life-threatening.
Each of these character must be played and plotted well, through the entire book. Their fates provide tension. You want to keep them around as long as possible.
For the last book, this also includes Neville. We need him around for the doubtful and unexpected twists in the finale.
Snape would normally be a B character, but for this last book, we also need him around for the twisty and unexpected finale. Malfoy has been around so long that we must expect him in the twisty ending, too.
Dumbledore. He's dead. But oh, you gotta expect at least one jedi moment. Come on folks, this is Dumbledore. Even dead, expect him to play some role.
In bridge, these are your aces. You've got to use them well.
These include Minerva, the Weasley family, Durslies, etc.
We've got lots of emotional investment in these characters. Expect these characters to come under threat alot. Expect them to hurt a whole lot. They've got to suffer so that Harry suffers. Expect misfortune to come to these characters, but few deaths. These characters are more useful alive than dead, as they provide tension and impetus for your lead characters. The deaths that do matter here need to count, enough to drive the characters onward.
These characters also provide a few surprises. Expect one from Minerva. (You gotta expect that that old witch has something surprising about her. Dumbledore made her #2 for a reason.)
In bridge, these are your face cards.
Expect C characters to buy it. This includes major characters from previous books. This includes all aurors. Also includes pets. You may like them, but dramatically, they are mostly useful in death. They provide you the dark shadows that you need. We need a steady stream of names that we know reminding us that there is TENSION. We need them dying just often enough to make us fear for the A and B characters.
In bridge, these are your non-scoring cards. Expect to lose them as you play. Their main use is helping the bad guys look bad.
You've heard them mentioned. They've shown up sometimes for color. Expect them to burn like deadwood. These guys are all "red shirts". Their deaths really don't matter. They are here to demonstrate that the bad guys are remorseless. Expect a steady stream of characters that you've forgotten falling dead.
Playing the Hand
You need to start out with death early. Voldemort needs to come out with a strong hand and run the board for a while. Death tells us the rule of the game.
The ending must be twisty. Gotta keep the reader guessing. That means that Draco, Snape, and Neville must live to the end. This means that Dumbledore's one-last-gift (whatever that may be) must also come into play at the end. So will his Phoenix. (I'd put good money on the Big D himself showing up.)
The ending is a long way away. We can't have the leads threatened all the time, but we do need them threatened enough to keep the book from dragging. Expect two encounter with Voldemort before the finale.
We must see Hogwarts. Hogwarts is an A character. The finale best takes place in Hogwarts. Where else is the finale to take place? You may be able to get the whole student body into that finale.
Expect a deus-ex-machina of some sort along the way. The deus-ex-machina will figure promenently in the ending.
Riddles. Must have riddles to solve. Feed the Hermione.
The character need a way to locate the hoarcruxes.
The villains must continue their victory, especially at the end, when the wizarding world has folded under Voldemort and his final victory is nigh.
Insert at least one major revalation or change in of each A character. Something "surprising". Yes, this means that Ron may actually be useful for something.
Hmmm. The middle looks like a disconnected collection of episodes. That's about normal for Rowling.