A/N: I know this one doesn't involve Harry directly, but I couldn't resist.
Chapter 14: Lupin solves Snape's dislike of him, Harry, and Neville.
Snape scowled as he went through the homework assignments piled in front of him. His scowl darkened as his office door opened and one of the two people in the castle he hated the most walked in. "Lupin," he said darkly. "To what do I owe this visit?" In answer, Lupin held up a small vial, which Snape recognised as Veritaserum, taken from his own stores. "I'm going to prove something to you, Severus. A better man would have put aside his schoolboy grudges years ago, but you clearly need convincing to do so."
With that, Lupin opened the vial and carefully trickled a small dose of the potion down his throat. When he next spoke, it was in the calm tone that Snape knew proved the potion had taken effect. "The incident in which you saw me transformed – I knew nothing about it until after the fact."
Snape's jaw dropped. "Why then," he asked once he recovered himself, "did you not tell me at the time?" "I knew that you would not have believed me." As the Veritaserum wore off, Lupin shook his head and blinked several times. "Incidentally, Severus, a piece of advice. Either remove your memories of James, or speak to Dumbledore and have him Obliviate you of them. You are allowing your hatred of James to spill over onto Harry, solely because he looks like his father – a father he does not remember! The only reason he acts towards you the way he does is because you started it!"
Snape's jaw dropped again, and Lupin turned to leave, delivering a final verbal blow as he did so. "Also, you no doubt heard that Neville Longbottom's Boggart was you, Severus. Might that not be linked to his performance in your class? The boy is scared of you, which makes him make mistakes out of nerves. You then ridicule him for the mistakes, thus making him feel worse. It is a vicious circle."
As his office door swung closed behind Lupin, Snape sat in thought. If he changed his behaviour, Potter and Longbottom might change theirs, and maybe even other Gryffindors…
For the next week, the main subject of discussion among the Gryffindor third years was why Professor Snape was being nice to them all of a sudden. Lavender Brown and Parvati Patil both spent a day in the hospital wing after Madam Pomfrey suspected them of having gone mad when they talked about him giving them points, within earshot of her.