"If you don't mind me asking, how -- how did you get out of Azkaban, if you didn't use Dark Magic?" 

"Thank you!" gasped Pettigrew, nodding frantically at her. "Exactly! Precisely what I --"

 But Lupin silenced him with a look. Black was frowning slightly at Hermione, but not as though he were annoyed with her. He seemed to be pondering his answer. 

"I don't know how I did it," he said slowly. "I think the only reason I never lost my mind is that I knew I was innocent. That wasn't a happy thought, so the dementors couldn't suck it out of me... but it kept me sane and knowing who I am... helped me keep my powers... so when it all became ... too much... I could transform in my cell... become a dog. Dementors can't see, you know...." He swallowed. "They feel their way toward people by feeding off their emotions.... They could tell that my feelings were less -- less human, less complex when I was a dog... but they thought, of course, that I was losing my mind like everyone else in there, so it didn't trouble them. But I was weak, very weak, and I had no hope of driving them away from me without a wand...." 

"But then I saw Peter in that picture... I realized he was at Hogwarts with Harry... perfectly positioned to act, if one hint reached his ears that the Dark Side was gathering strength again...."

 Pettigrew was shaking his head, mouthing noiselessly, but staring all the while at Black as though hypnotized.

 "... ready to strike at the moment he could be sure of allies... and to deliver the last Potter to them. if he gave them Harry, who'd dare say he'd betrayed Lord Voldemort? He'd be welcomed back with honors.... 

"So you see, I had to do something. I was the only one who knew Peter was still alive...." 

Harry remembered what Mr. Weasley had told Mrs. Wealsey. "The guards say he's been talking in his sleep... always the same words... 'He's at Hogwarts.'"