Crucio

From the Order of the Phoenix:

Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, 'Crucio!'

Bellatrix screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had – she was already back on her feet, breathless, no longer laughing. Harry dodged behind the golden fountain again. Her counter-spell hit the head of the handsome wizard, which was blown off and landed twenty feet away, gouging long scratches into the wooden floor.

‘Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it - righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson –’

There is something beyond just casting these spells (I've never heard of Expelliarmus not working because the caster wasn't really sure they wanted to disarm someone or Alohomora not working because you weren't sure you want to find what's behind the door). They aren't as effective if you don't mean it.

Imperius curse:

“They’re Imperiused,’ he added, in response to Hermione and Ron’s confused queries about Travers and Bogrod, who were both now standing there looking blank. ‘I don’t think I did it strongly enough, I don’t know …’

And another memory darted through his mind, of the real Bellatrix Lestrange shrieking at him when he had first tried to use an Unforgivable Curse: ‘You need to mean them, Potter!”

We can see that perhaps some level of skill or desire is needed here - the memory of the earlier event seems to imply it is a desire or malice that is needed.

Avada Kadavra:

“Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nose-bleed. But that doesn’t matter. I’m not here to teach you how to do it.”

And Snape uses it to kill Dumbledore - I doubt he really wanted him to die - so Avada Kadavra seems to take power, rather than desire. Voldermort is spoken of as one of the most powerful wizards and uses Avada Kadavra frequently throughout the first and second wizarding wars. So it seems power (and perhaps practice) are factors with this one.

So why don't they use them?

The three unforgivable curses are forbidden, it is unlikely that most, especially those on the 'good' side, have practiced them. When your enemy is bearing down on you do you want to try something you know – like Expelliarmus – which can save your life in this moment or do you attempt an unpracticed spell that relies on power you may not have or a desire you don't truly feel but could, if successful, defeat your enemy. I imagine most would go for winning in that moment.

Another perspective comes from looking at our own world. Introduce a more dangerous weapon to any arsenal – be that gang crime or international relations – and soon everyone is using it. You see it in gang crime, once you know your enemy may be carrying a knife you don't leave your house without one and if the US fires a nuclear missile at Russia (or vice versa) and the response was a nuke in return... how does the world fare then?

This part is conjecture but perhaps at the back of every witch and wizard's mind is the awareness that there aren't so many of them and pulling out avada kadavra as a first resort will only encourage the other side to do the same.

This seems to be due to some sort of "morality" that some of the good-guy wizards believe in, most notably Harry and Dumbledore. We know Dumbledore particularly avoided killing, as Harry stated at the end of Deathly Hallows:

"Master of death, Harry, master of Death! Was I better, ultimately, than Voldemort?"

"Of course you were," said Harry. "Of course – how can you ask that? You never killed if you could avoid it!"

And Harry had a bit of a heated exchange with Lupin after the battle at the beginning of Deathly Hallows. Some of Harry's choice lines from there:

"I saw Stan Shunpike.... You know, the bloke who was the conductor on the Knight Bus? And I tried to Disarm him instead of – well, he doesn't know what he's doing, does he? He must be Imperiused!"

"We were hundreds of feet up! Stan's not himself, and if I Stunned him and he'd fallen, he'd have died the same as if I'd used Avada Kedavra! Expelliarmus saved me from Voldemort two years ago,"

"So you think I should have killed Stan Shunpike?" said Harry angrily.

"I won't blast people out of my way just because they're there," said Harry, "That's Voldemort's job."

Although in that instance Harry was specifically talking about Stan Shunpike, whom he believed to be innocent, it seems from Lupin's responses (as I argued here) that Harry was against severely attacking even actual Death Eaters.

For whatever reason, Harry, Dumbledore, and perhaps others seem to believe that it is wrong to kill Death Eaters — even if that gives the Death Eaters have future opportunities to kill others.

In truth, though, for all his experience and D.A. lessons Harry is still a novice. He doesn't actually have a repertoire of many damaging spells. Stupefy and Petrificus Totalus might be a step up from Expelliarmus, but under normal circumstances the Death Eater can be freed from those within moments. Harry has never demonstrated the ability to perform Avada Kedavra, and his Crucios are mild at best. The only other powerful curse that we really see him use is Sectumsempra, and even that would not necessarily incapacitate an enemy, and could be counteracted by a knowledgeable wizard. So even if Harry did not have these moral compunctions there is not all that much more that he could have done. Harry is simply overrated. To quote our favorite Potions Master from the end of Deathly Hallows (my emphasis):

"– mediocre, arrogant as his father, a determined rule-breaker, delighted to find himself famous, attention-seeking and impertinent –"

Bellatrix's answer touches on an extremely important point, although I don't think it is emphasized enough: wizards are people, and most people are conditioned from childhood not to hurt other people. It's called civilization. It can be very difficult to cast aside.

The classic treatment of the subject is S.L.A. Marshall's "Men Against Fire", which dealt with the behavior of US infantry soldiers in actual combat. There has arisen very considerable doubt that his numbers are honest, but he concluded that in general in a company under attack only 15% of the soldiers would actually use their weapons, with as much as 25% for elite units "under intense local pressure", which is mil-speak for the situation going to Hell in a handbasket and a really desperate fight. He also remarked on the fact that the same names kept cropping up for a unit fighting multiple engagements, suggesting that the same small proportion (less than 15%) were the mainstays of the unit when the bullets were actually flying.

It may be unrelated, but a post-D-Day study by Swank and Marchand concluded that after 90 days of combat, 98% of the physical survivors were psychological casualties, with the most common diagnosis of the unaffected being "aggressive psychopathic personalities". This does suggest that some of the best soldiers, in terms of combat effectiveness, were not what you'd call nice people.

Unless wizards are somehow qualitatively different from muggles psychologically, it would make sense that most would instinctively shrink from using the effective (unforgivable) spells, even if it meant that they would suffer for it. And, given the importance of whole-hearted intent in casting the powerful spells, any internal conflict would make attempted use less likely to succeed. And the same personality defects which would cause a person to side with Voldemort would tend to make his followers more likely to be able to use the unforgivable spells, at least as compared with the good guys.

EDIT - And about internal conflicts and effectiveness. WWI saw the recognition of what Freud dubbed "conversion hysteria" on a widespread basis among soldiers. This occurs when emotional stress manifests itself as paralysis of a body part with no organic cause. In the case of soldiers, it showed up as paralysis of the hand or even of the trigger finger. If the stress of trying to kill someone can produce such gross muggle effects, it certainly seems reasonable that a similar condition could occur among wizards, which would make casting unforgivable spells extremely difficult or impossible.

As a personal illustration, as a Vietnam helicopter door gunner, the first time I was called upon to fire with a visible target, I had to deal with the fact that for several seconds my trigger finger simply would not move. It was really the damndest sensation, looking at my hand which steadfastly ignored the commands which my brain was sending. This only lasted for a few seconds, but the experience left quite an impression on me.

There are several reasons.

Many factors contribute to why most wizards don’t use Unforgivable Curses in battle, and each individual wizard who chooses not to use them may base their choice on one or a combination of reasons.

Unforgivable Curses are illegal.

Using the Unforgivable Curses is illegal - using any one of them is enough to get a life sentence in Azkaban. Even wizards who are capable and willing to use them may not if they consider the possibility of being sent to Azkaban for life over their use.

“Now … those three curses – Avada Kedavra, Imperius and Cruciatus – are known as the Unforgivable Curses. The use of any one of them on a fellow human being is enough to earn a life sentence in Azkaban.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)}}

That’s likely a very strong deterrent to using them, even in serious situations, since no one wants to go to Azkaban. It also would work on some wizards who’d have no other problems with using Unforgivable Curses, because the consequences are extreme.

They’re difficult to cast.

Additionally, only wizards with a certain level of skill and power can cast Unforgivable Curses. As Snape tells Harry, casting Unforgivable Curses requires both nerve and ability.

“Cruc—’ yelled Harry for the second time, aiming for the figure ahead illuminated in the dancing firelight, but Snape blocked the spell again; Harry could see him sneering.

‘No Unforgivable Curses from you, Potter!’ he shouted over the rushing of the flames, Hagrid’s yells and the wild yelping of the trapped Fang. ‘You haven’t got the nerve or the ability –”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 28 (Flight of the Prince)}}

“Nerve” likely refers to the desire to use the Curse, something that’s particularly important with the Cruciatus Curse. Crucio can cause varying degrees of pain, and the more you want to cause pain, the more effective it’ll be. A Cruciatus Curse that doesn’t have that desire behind it will be ineffective, so wouldn’t help much in a battle.

“Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, ‘Crucio!’

Bellatrix screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had – she was already back on her feet, breathless, no longer laughing. Harry dodged behind the golden fountain again. Her counter-spell hit the head of the handsome wizard, which was blown off and landed twenty feet away, gouging long scratches into the wooden floor.

‘Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it – righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson -”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

Avada Kedavra has only two possible effects - it either works or it doesn’t, it can’t kill someone any more dead if cast more skillfully than necessary, but it takes a certain level of skill and power to cast it effectively. If it fails, then it fails entirely.

“Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nose-bleed. But that doesn’t matter. I’m not here to teach you how to do it.”

_{^{- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)}}

For a wizard that’s not skilled enough to use the Unforgivable Curses, it’d be more effective to cast a spell that they’d be successful at using that attempting to use an Unforgivable Curse unsuccessfully.

They’re also considered immoral by some.

Furthermore, some wizards wouldn’t want to use Unforgivable Curses at all, even if using Unforgivable Curses wasn’t illegal and they were skilled enough to use them. Dumbledore avoided using Dark magic entirely.

“I know you haven’t,’ said Professor McGonagall, sounding half-exasperated, half-admiring. ‘But you’re different. Everyone knows you’re the only one You-Know – oh, all right, Voldemort – was frightened of.’

‘You flatter me,’ said Dumbledore calmly. ‘Voldemort had powers I will never have.’

‘Only because you’re too – well – noble to use them.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 1 (The Boy who Lived)}}

It’s very rare to see a member of the Order use an Unforgivable Curse, which is likely at least in part to a desire not to use Dark magic. Moody, a member of the Order of the Phoenix, also had a similar set of beliefs as an Auror - he avoided killing, and likely Unforgivable Curses as well.

“I’ll say this for Moody, though, he never killed if he could help it. Always brought people in alive where possible. He was tough, but he never descended to the level of the Death Eaters.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

Sirius Black, another member of the Order, thought Barty Crouch Sr. went too far in his attempts to capture Death Eaters using violence against them, which included allowing Aurors to kill and authorizing them to use the Unforgivable Curses.

“Crouch’s principles might’ve been good in the beginning – I wouldn’t know. He rose quickly through the Ministry, and he started ordering very harsh measures against Voldemort’s supporters. The Aurors were given new powers – powers to kill rather than capture, for instance. And I wasn’t the only one who was handed straight to the Dementors without trial. Crouch fought violence with violence, and authorised the use of the Unforgivable Curses against suspects.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

Harry, Ron, and Hermione all seem very reluctant to kill Antonin Dolohov and Thorfinn Rowle, both notorious Death Eaters. Ron brings up the idea, but is clearly relieved when Harry rejects the idea. Hermione shuddered at the thought of it.

“What are we going to do with them?’ Ron whispered to Harry through the dark; then, even more quietly, ‘Kill them? They’d kill us. They had a good go just now.’

Hermione shuddered and took a step backwards. Harry shook his head.

‘We just need to wipe their memories,’ said Harry. ‘It’s better like that, it’ll throw them off the scent. If we killed them, it’d be obvious we were here.’

‘You’re the boss,’ said Ron, sounding profoundly relieved. ‘But I’ve never done a Memory Charm.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 9 (A Place to Hide)}}

So, that’s yet another reason why the average wizard wouldn’t use Unforgivable Curses - some consider it wrong. Also, those who consider it wrong to use Unforgivable Curses are likely to be less effective at casting them, even if they tried, as they mightn’t be able to put enough power behind the Curses to cast them well.

As you say, the interesting question is why any non-evil wizards ever use anything other than Stupefy in a combat situation, given that knocking someone unconscious seems strictly better than temporarily disarming them, immobilising them or slowing them down. Here are a few possible answers, largely based on evidence from the wiki (primarily pages for the relevant spells):

Casting spells seems to require concentration/mental energy (see wiki page on non-verbal skills, and the fact that we don't see people cast them as fast as they can say Stupefy/perform gestures). If less powerful spells demand less mental energy (seems plausible since they are taught before more powerful ones), using e.g. Expelliarmus over Stupefy might allow a higher "rate of fire".

There are also some tactical advantages to certain spells:

• Stupified people can be revived by allies, so Expelliarmus could be better than Stupefy if it would deprive someone of their wand for longer than it would take them to be Rennervated following a Stupefy hit.


• Various non-humans are immune to Stupefy.


• According to the wiki, Expelliarmus can also reflect enemy spells which is obviously useful.


• Different spells might have different properties with respect to shields and dodging. For instance, Petrificus Totalus doesn't seem to involve firing a beam, so it might be more difficult to block/dodge than Stupefy

Finally, although a single Stupefy is non-lethal, multiple simultaneous hits can apparently kill. So that's another reason for avoiding it if you're a goodie.

I infer that because the wizards are usually in tense situations the easiest spells to pronounce and the spells that they are certain will work, pop into their head first.
To support my answer,"Harry is fond of using the spells: “Expecto Patronum,” “Stupefy,” and “Expelliarmus”".The spell that pops up to me most there is "Stupefy" a spell that is easy to pronounce and surprisingly wont to any large amount of damage to the victim. "Stupefy" was taught to "Dumbledore's Army"in book 5, therefore being a spell that Harry would like the "Army" to use while in battle and Harry would not have chosen this particulate spell if he himself did not prefer it.
Here are just several examples of harry using this spell in "tense situations.

-Death Eater; During the early stages of the Battle of Hogwarts Harry used this spell on a Death Eater.

Lucius Malfoy; Battle of Malfoy Manor

Fenrir Greyback; Battle of Malfoy Manor

Thorfinn Rowle; During a skirmish at the Luchino Caffe in 1997.(MOVIES)

Viktor Krum; During the Third Task of the Triwizard Tournament to save Cedric Diggory from the Cruciatus Curse Krum casted under the influence of the Imperius Curse.(MOVIES,BOOK)

As you to have noticed each of these times where Harry has used the spell stupify has been in an extremely tense situation and the only reason for this is due to harry being familiar with the spell, the spell being easy to pronounce and harry knowing that the spell will not fail him.

I rest my case