ETA: gjm is right: Voldemort himself says the target must be either willing or too weak to resist him. So Voldemort can’t possess most random people, and certainly not Dumbledore.
If Voldemort could possess Dumbledore, he could have done so earlier and simply given himself the Stone, as well as the Elder Wand and any other interesting gadgets Dumbledore has. He’d have made Dumbledore teach him all the spells he knows, too, if those survive possession. And he’d have used Dumbledore to attack by surprise Flamel or anyone else if he wanted to.
In the least convenient world, possessing someone doesn’t give Voldemort access to any of their previous memories or spells, it just wipes the mind and takes over the body. Then it might be hard to possess Dumbledore and use him to get the Stone. But he could still take out Dumbledore, cause Flamel to remove the Stone from Hogwarts, and then possess Flamel while he has the Stone.
Possessing anyone in the world (or at least, any wizard other than Harry) just by making them touch any object you prepared beforehand is so overpowered that if it’s true, the only possible reason Quirrel is still plotting anything or playing games or appearing to ever be in danger, is that he’d be bored otherwise.
If Voldemort can possess Dumbledore at will and chooses not to, the only reason I can think of is that it would make the game too boring, like he said about the Wizarding War. This strikes me as… inadequate. With the Stone involved, Voldermort is playing for real stakes, and waiting for the end of the year increases the chance something will go wrong, weakens the Quirrel body, and eventually forces Voldemort to operate on the last day of the school year, when others might guess something important would happen.
(Edited:) But why wouldn’t Voldemort be able to possess Dumbledore, or anyone else in the world, other than Harry? He claims the only requirement is to have the target touch a Horcrux 2.0. This seems fairly easy: to give make sure your target will touch a Horcruxed coin, you can just toss them one and they will catch it by reflex, as an earlier chapter pointed out. And you can horcrux the doorknob of their office, a chair they’ll sit on in a public park, etc. It may be, however, that since Voldemort has only had this ability for a few months, during which Dumbledore has been on high alert, he hasn’t been able to accomplish this yet.
I just realized Voldemort probably cast the revised Horcrux on Roger Bacon’s diary with wordless magic:
The book vanished back into Professor Quirrell’s robes, and he rose up from his chair. Professor Quirrell started to walk over to the door -
and staggered, suddenly lurching into the wall.
“It’s all right,” said Professor Quirrell’s voice, which suddenly sounded a lot weaker than usual. “Sit down, Mr. Potter, it’s just a dizzy spell. Sit down.”
[...]
“That, Harry Potter, is the diary of Roger Bacon.”
Harry almost fainted.
Nestled up against the wall, where Professor Quirrell had stumbled, glistened the crushed remains of a beautiful blue beetle.
Or he could have just cast the spell at any other time of his choosing prior to giving Harry the diary. For that matter, he could have not done it at all, since there’s no actual benefit to making the diary a Horcrux—he already has all the Horcruxes he needs, and it’s not like he can possess Harry anyway.
ETA: gjm is right: Voldemort himself says the target must be either willing or too weak to resist him. So Voldemort can’t possess most random people, and certainly not Dumbledore.
If Voldemort could possess Dumbledore, he could have done so earlier and simply given himself the Stone, as well as the Elder Wand and any other interesting gadgets Dumbledore has. He’d have made Dumbledore teach him all the spells he knows, too, if those survive possession. And he’d have used Dumbledore to attack by surprise Flamel or anyone else if he wanted to.
In the least convenient world, possessing someone doesn’t give Voldemort access to any of their previous memories or spells, it just wipes the mind and takes over the body. Then it might be hard to possess Dumbledore and use him to get the Stone. But he could still take out Dumbledore, cause Flamel to remove the Stone from Hogwarts, and then possess Flamel while he has the Stone.
Possessing anyone in the world (or at least, any wizard other than Harry) just by making them touch any object you prepared beforehand is so overpowered that if it’s true, the only possible reason Quirrel is still plotting anything or playing games or appearing to ever be in danger, is that he’d be bored otherwise.
If Voldemort can possess Dumbledore at will and chooses not to, the only reason I can think of is that it would make the game too boring, like he said about the Wizarding War. This strikes me as… inadequate. With the Stone involved, Voldermort is playing for real stakes, and waiting for the end of the year increases the chance something will go wrong, weakens the Quirrel body, and eventually forces Voldemort to operate on the last day of the school year, when others might guess something important would happen.
(Edited:) But why wouldn’t Voldemort be able to possess Dumbledore, or anyone else in the world, other than Harry? He claims the only requirement is to have the target touch a Horcrux 2.0. This seems fairly easy: to give make sure your target will touch a Horcruxed coin, you can just toss them one and they will catch it by reflex, as an earlier chapter pointed out. And you can horcrux the doorknob of their office, a chair they’ll sit on in a public park, etc. It may be, however, that since Voldemort has only had this ability for a few months, during which Dumbledore has been on high alert, he hasn’t been able to accomplish this yet.
No. They must either consent or be too weak to resist. It seems unlikely that either Dumbledore or Flamel would meet either condition.
You’re right. I’ll amend my comment.
I just realized Voldemort probably cast the revised Horcrux on Roger Bacon’s diary with wordless magic:
[...]
http://hpmor.com/chapter/26
Or he could have just cast the spell at any other time of his choosing prior to giving Harry the diary. For that matter, he could have not done it at all, since there’s no actual benefit to making the diary a Horcrux—he already has all the Horcruxes he needs, and it’s not like he can possess Harry anyway.
Turns out I was quite wrong anyway.