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We meet the dementors in 'Prisoner of Azkaban'. I've noticed that dementors are implied to be able to communicate 'off-screen' so to speak, but do not do so on-screen.
In the book, the first time a dementor appears is on the Hogwarts Express. Lupin says "None of us is hiding Sirius Black under our cloaks", but they don't listen to him. It's unclear if they even understand him.
Similarly, they repeatedly violate instructions throughout the book, maybe knowingly, but at no point is it indicated that they actually comprehend what others are saying.
Later, they are described as having no eyes and "something like a mouth", which they use to perform the kiss. It is unclear if they do or even can use it to talk. They are also described as being able to sense emotions. Whether they have other senses like hearing isn't clear.
Now, the 'off-screen' part. Fudge, on multiple occasions says "I've got to talk to the dementors", like after Black escapes from Hogwarts. (How? How does he talk to them?)
And the totally troubling part: In the beginning, Arthur Weasly tells his wife that guards at Azkaban (which I assume to be dementors, since there is no evidence of anyone else guarding Azkaban), have stated that Black keeps muttering "He's at Hogwarts." That implies they can hear, understand and communicate to others what Black said.
So which one is it? Can they talk or not. If they can, why don't they listen to Lupin or even Dumbledore?
PS - It's been a while, but I have read the entire series. Don't worry about spoilers.