Aye they can, but that should not be abused in stories. The Unforgivables are the sublime peak of magic. The most powerful, feared spells ever to be created/discovered. The gift of Mars to the wizards. The bane of Dumbledore, the foundation upon which the Magical Roman Empire was built.

If someone starts casting a Killing Curse at you, the initial instinct is to cower in fear, having heard of the rather permanent effect. Someone more trained and braver could stand and fight, but the reaction time needed to transfigure/conjure a rather large physical shield would be inhuman. Dumbledore should be able to do it. Harry should have been able to do it. Voldemort should be a master.

I don't think anyone under Alastor Moody should stand a chance however. Imagine reading about Mollywobbles conjuring slabs of marble to block Bellatrix' Cruciatus Curses. It is unholy and blasphemous, considering the woman's grasp on magic should end at potato peeling charms and cleaning charms.

 

To add something about the 'long term' Imperius curse, I think that the logical explanation for that, as the caster isn't in permanent contact with his victim, is that when he uses the curse he can, if given enough time, plant orders in the victim's subconscious and hide them even from the person who's been cursed.

For exemple you could have someone 'programmed' to, in case of Death Eater attack (which act as the trigger for the hidden command) use their wand to cast an AK on the nearest person. (I know this is a bad exemple because, does the person would be able to use AK?)

This could explain the 'long term' 'short term' variation of the Imperius curse : If the caster have enough time he can put some triggers, false memories, orders and make the victim forget she has been cursed and she will later act accordingly.

 

Didn't I just discuss that Unforgivables probably can't be powered by emotion, but are rather powered by intent?

Am I just writing random gibberish here? I ask because the only response I got was something that in no way followed from my earlier post.

Concerning the nature of the Imperius curse in long-term situations, I don't understand the assumption that it would be any different from the standard situation. It's hardly any different from, say, activating a sleeper agent with instructions to await further orders. Rowling's choice of demonstrations of the Imperius curse early in the books is... well, a poor one. She displays it as a curse which controls action, when it's shown to be primarily used for long-term effects, which implies that it bends the victim's will and more or less enslaves it to the caster's.

Narion