What are the requirements for Imperio?

Question

For using the Cruciatus Curse, the caster must really feel hate towards the recipient.

Is there such a requirement for using the Imperius Curse? Is it available to only dark and powerful wizards?

Answer 1

No, Imperio doesn’t seem to have any emotional requirements.

Harry casts Imperio twice in Gringotts, without particularly strong emotions against the victims. It’s also important to remember, these are his first and second times ever casting the Imperius Curse, so he had no practice or experience casting it. He’s fairly successful, despite that.

“Act now, act now,’ whispered Griphook in Harry’s ear, ‘the Imperius Curse!’

Harry raised the hawthorn wand beneath the Cloak, pointed it at the old goblin and whispered, for the first time in his life, ‘Imperio!’ A curious sensation shot down Harry’s arm, a feeling of tingling warmth that seemed to flow from his mind, down the sinews and veins connecting him to the wand and the curse it had just cast.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

Harry casts it without thinking one of those times. He certainly wasn’t channeling any strong emotion then, he was acting on reflex - and it worked fairly well, though it was only his second one.

“Harry acted without thinking: pointing his wand at Travers, he muttered, ‘Imperio!’ once more.

‘Oh, yes, I see,’ said Travers, looking down at Bellatrix’s wand, ‘yes, very handsome. And is it working well? I always think wands require a little breaking in, don’t you?”
_{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

He does later think the reason his Imperio wasn’t particularly strong might be because he didn’t ‘mean it’ enough, but we don’t know if that’s true and Harry certainly isn’t an expert in the Dark Arts. There’s no other reference to needing to ‘mean’ the Imperius Curse, and Harry being able to cast it fairly successfully his second time using it suggests this probably isn’t true.

It does take a certain amount of skill, and likely a strong will.

Though it’s unclear exactly how much skill is required to cast the Imperius Curse, it presumably would take a certain amount of skill to cast it properly. In addition, casting it effectively may also involve strength of will somewhat - having a stronger will may help in casting it, since the Imperius Curse involves subverting another’s and replacing it with your own.

“Imperio!’ Harry said again; his voice echoed through the stone passage as he felt again the sense of heady control that flowed from brain to wand. Bogrod submitted once more to his will, his befuddled expression changing to one of polite indifference, as Ron hurried to pick up the leather bag of metal tools.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

The exact level of magical skill required isn’t known. Though it’s considered Dark magic, casting it successfully isn’t limited to “Dark” wizards, since we know Harry casts it fairly successfully on his first try, and McGonagall casts it as well. The wizards we see casting it (the Dark Lord, Barty Crouch Jr., Barty Crouch Sr., McGonagall, Harry) all seem to be somewhat above average in skill.

Answer 2

It seems likely that the same requirement exists for the Imperius Curse that exists for the Cruciatus Curse. When Harry uses the Imperius Curse for the first time in Gringotts, we see the following:

Deathly Hallows Chapter 26

“We’re in trouble; they suspect,” said Harry as the door slammed behind them and he pulled off the Invisibility Cloak. Griphook jumped down from his shoulders; neither Travers nor Bogrod showed the slightest surprise at the sudden appearance of Harry Potter in their midst. “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!”

Here Harry seems to apply the same standard to both curses.

This makes sense, as Bellatrix's statement itself referred to the Unforgivable Curses in general:

Order of the Phoenix Chapter 36

"Never used an Unforgivable Curse before, have you, boy?" she yelled. She had abandoned her baby voice now. "You need to mean them, Potter!

Similarly, a comment from Snape at the end of Half-Blood Prince seems to equate all the Unforgivable Curses:

"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 –"

Answer 3

in Goblet Of Fire, when Mad Eye Moody

impersonated by Barty Crouch, Jr.

attempted to cast Imperio on Harry, Harry managed to resist the curse. Later in the same book we find out that other characters also managed to resist and even break Imperius curse.

Thus the only requirement mentioned in the books (indirectly) is the willpower of the caster. Finally, it all comes down to a willpower combat between the caster and the victim.