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Competent wizards, (as Voldemort clearly is) can effectively cast spells without speaking. Dumbledore does this to reveal Harry under his invisibility cloak:

J.K. Rowling: Dumbledore, who could perform magic without needing to say the incantation aloud, was using ‘homenum revelio’

Why does Voldemort always say 'Avada Kedavra' when casting the spell?

marked as duplicate by Mithrandir, BMWurm, Au101, FuzzyBoots, Bamboo Apr 9 '17 at 18:19

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In most cases, it’s not worth the extra effort to cast it nonverbally.

It's made clear that casting the same spells nonverbally takes much more effort than it would if the person casting it says the words, even for a powerful wizard like Lord Voldemort. Nonverbal spells are consistently said to be much harder to cast than their spoken counterparts.

The first quote that I could think of mentioning the difficulty of nonverbal spells is this one from when Snape was teaching them in Harry's sixth year:

“Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some ...lack.”
- Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)

In most cases, if Lord Voldemort is casting a Killing Curse at someone, they're soon to be dead. When fairly sure that his target will be dead soon, it wouldn't make sense for him to waste the energy on casting Avada Kedavra as a nonverbal spell.

He can indeed cast it nonverbally - he casts it nonverbally at the Ministry several times when dueling Dumbledore. This makes sense, as Dumbledore is a powerful opponent he’d want to give himself every advantage he possibly could against. Dumbledore was very skilled and the only one the Dark Lord ever feared, he wasn’t going to take a duel with him lightly.

“But even as he shouted, another jet of green light flew at Dumbledore from Voldemort’s wand and the snake struck –

Fawkes swooped down in front of Dumbledore, opened his beak wide and swallowed the jet of green light whole: he burst into flame and fell to the floor, small, wrinkled and flightless.”
- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)

Just because he can do it nonverbally doesn't mean it's always the best choice. In most cases where he says the words Avada Kedavra, it's not while facing someone like Dumbledore. It's when he's using it on people who don't have much of a chance against him that he says the words. For example, when he killed the Potters, he said the words because the targets of his curse weren't going to be able to stop it, even if they knew it was coming.

  • ...[W]hen he killed the Potters, he said the words because the targets of his curse weren't going to be able to stop it, even if they knew it was coming.... Or so he thought. – trysis Apr 9 '17 at 16:34
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    @trysis The Dark Lord had no reason to believe that his Killing Curse would not work on a mere infant. – Bellatrix Apr 9 '17 at 18:22
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    Miss Bella as I told you too you do sound like her - quite well. But I am pretty sure Voldemort in OotP does cast it wandlessly. Check the duel between Dumbledore for that. – Pryftan May 27 '18 at 23:54
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    @Miss Bella Plus he probably wanted to show off a bit. You know..before he kills him. Well of course that's perhaps not the way it is exactly because he did fear Dumbledore and knew he would be a formidable foe (only that Dumbledore doesn't seek to kill); but he does seem to like to show his power. As all people like he would. I would say that he usually does say it - almost always? - but in some cases not like in this duel with Dumbledore. Anyway yes I think you should add it to the answer (saying that because I noticed you didn't get to it yet). – Pryftan May 28 '18 at 18:31
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    @Pryftan Yes, it’s possible he wanted to show off a bit as well, and he also did fear Dumbledore which is why he never actually sought out a confrontation with him. He usually says it, but with Dumbledore he’d be more wary. I added in a clear case of the Dark Lord nonverbally casting Avada Kedavra, thanks! – Bellatrix Jun 2 '18 at 18:27
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The advantage of nonverbal casting is that you don't relay any info to your opponent. If you start shouting an incantation, the opponent has a split second to parry your spell or block it using a specific counterspell.

The Avada Kedavra curse, however, cannot be blocked, so you gain nothing by casting it non-verbally. It doesn't matter if you telegraph to your opponent you are about to cast it, there's nothing they can do to block it. So you're just making the spell more complex to cast for no real gain.

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    If the opponent doesn't know what you're casting, they might not know it's unblockable, giving you the advantage. – neverendingqs Apr 8 '17 at 21:16
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    Although the curse is unblockable, it's not unshieldable: remember what Voldemort did in the Ministry when he was expecting Dumbledore to attempt to kill him. Also you can hide, as Harry hid from Cruciatus trying to get to the portkey in GoF. – Ruslan Apr 9 '17 at 8:48
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    Or presumably apparate away. Of course, if Voldermort is pointing his wand at you, you probably don't need to hear the spell before deciding you need to be somewhere else. – delinear Apr 10 '17 at 10:31

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