Flight of the Fat Lady
Chapter 8 of Harry Potter and the Prisoner of Azkaban: Flight of the Fat Lady
Defence Against the Dark Arts is now the most interesting and popular class. Although Slytherins complain about Professor Lupin's patched robes, everyone else enjoys the lessons and actually finds them practical. Unfortunately, this is their only fun class. Potions is dreadful, particularly since Professor Snape heard about Neville turning the Boggart into a simulacrum of him dressed in an old woman's clothing. Snape now bullies Neville mercilessly in class. Divination is almost as bad, with Professor Trelawney frequently predicting Harry's death and looking at him tearfully. Care of Magical Creatures is also just as bad now that Hagrid has lost confidence and only teaches about Flobberworms.
On a brighter note, Quidditch is starting up, and Oliver Wood has had the team practicing since early October. Wood, now a seventh-year, sees this as his last chance to win the Quidditch cup for Gryffindor. He feels that it should have been theirs for the past two years, but Harry was out of commission at the critical match in his first year, and his second year, the Quidditch Cup had been canceled.
A notice is posted for the first Hogsmeade weekend. Ron suggests that Harry, who lacks his guardian's permission, should ask Professor McGonagall. As they are discussing this, Hermione's cat, Crookshanks, attacks Scabbers, Ron's pet rat. Ron is still upset the next day, and barely speaks to Hermione.
The Trio, on their way to Transfiguration the next day, find Lavender crying over news she has received: her pet rabbit Binky has been killed by a fox. She and Parvati connect this with a prediction made by Professor Trelawney on the first day of Divination class, and seem to feel Hermione is being unfeeling when she unsuccessfully tries to reconcile the exact wording of Trelawney's prediction with what has actually happened.
At Ron's urging, Harry asks Professor McGonagall for permission to visit Hogsmeade with the other students, but she refuses to bend the rules that far. Harry must resign himself to staying behind. Percy only makes things worse with his over-pompous attempts to console Harry.
Wandering aimlessly around the castle on Saturday, Harry runs into Professor Lupin. He invites Harry into his office for tea and to see the Grindylow that he has just received for their next class. When Harry asks why he was prevented from facing the Boggart, Lupin explains he was concerned it would turn into Lord Voldemort and terrify the class. Harry, mildly surprised that Lupin uses Voldemort's name, is somewhat mollified by this explanation. He says his first thought was Voldemort, but then realized he was more frightened by Dementors. Lupin is impressed that Harry is more afraid of fear than an actual person. They are interrupted by Professor Snape who delivers a steaming goblet to Lupin. After Snape leaves, Harry, worried about the possible contents of the goblet, warns Lupin that many believe Snape would do anything to get the Defence Against the Dark Arts position, but Lupin drinks the potion nevertheless.
Ron and Hermione return with many stories about Hogsmeade. They are also amazed that Lupin would dare to drink anything prepared by Snape. Lupin attends the Hallowe'en Feast that evening, but Harry notices he looks unhappy. He observes Snape watching Lupin rather more intently than usual. Students head back to Gryffindor Tower, but there is a jam at the entrance; the Fat Lady is missing, her portrait slashed. Professor Dumbledore is summoned, and he questions Peeves, who says Sirius Black shredded the painting.
As the battle between Crookshanks and Scabbers escalates, Ron and Hermione's relationship becomes more strained. The readers can see, but Hermione evidently does not, that Crookshanks is deliberately targeting Scabbers, though why is unknown. Hermione seems adamant that cats attacking small animals is normal behavior, but we can see that Crookshanks has shown no interest in other pets such as Trevor, Neville's toad. Why has he become fixated only on Scabbers?
Considering Draco's comments in the previous chapter that Harry should want revenge against Sirius Black, a correlation can be made as to why authority figures such as Fudge (in Chapter 3) and McGonagall (here) deny Harry permission to visit Hogsmeade, and why Mr. Weasley attempted to procure Harry's promise not to search for Black. Although the pattern seen here would strongly suggest that Black has done something that would inspire a confrontation, Harry seems unable to tie these connections together.
From the discussion with Professor Lupin, we see Lupin's concern for Harry's emotional well being and also for the entire class. Lupin perhaps is more concerned about the students than any other teacher, with the possible exception of Professor Dumbledore. The Slytherins, meanwhile, are behaving as normal, making aspersions about Lupin's patched robes, obviously more impressed by a person's superficial outer appearance than with their inner character and abilities.
Harry is certainly wondering why Snape would deliver a potion to Lupin, though it is unknown what the cup contains. However, it must be something other than a mere drink for Snape to personally deliver it to Lupin; and Lupin's comment that sugar would ruin its effects indicates also that it has a magical component. While Harry may suspect it contains poison, that seems unlikely, even considering Snape's animosity toward Lupin; Snape's willingness to be seen presenting the goblet to Lupin strongly implies, if not innocence, at least inability to blame Snape for anything that happens to Lupin. What Harry has evidently forgotten is that Snape is as bound by the laws of the world as anyone else, and if he acts with malicious intent, he will suffer the consequences. Lupin knows that Snape cannot openly poison him, so feels safe in ignoring Harry's warning, but there must be some reason as to why Snape has brewed this for Lupin.
Sirius Black has breached the castle's supposedly impenetrable security as well as bypassing the vicious Dementors, making Hogwarts seem far less secure than we have been led to believe. The reader may suspect that Black could have broken into Hogwarts in the same manner in which he escaped from Azkaban, though we have no idea how he accomplished that.
Also, possible reasons can be seen for Hermione's disdain for Divination. It is mentioned in the Chapter 6, Talons and Tea Leaves, analysis that Professor Trelawney uses fortune-teller's tricks. One was a deliberately-vague "prediction" given to Lavender Brown, "That thing you are dreading. It is going to happen on Friday the sixteenth of October." On October 16th, Lavender receives word that a fox killed her pet bunny, Binky. Hermione questions how this actually fits with the prediction by recalling exactly what it said. Was the bunny's death even a surprise or was Lavender expecting it? Did it happen on the 16th, or is it only on the 16th that she had heard about it? Hermione apparently surmises that Lavender has rationalized her experience to match the prediction. Lavender and Parvati Patil, however, now seem to almost worship Professor Trelawney. Throughout the remainder of the book, these two spend much spare time hanging out in the Divination classroom.
- Why does McGonagall refuse to give Harry permission to visit Hogsmeade?
- What is Lupin's explanation to Harry about why he prevented Harry from facing the Boggart? Is he telling the truth?
- Why does Harry warn Lupin against drinking Snape's potion? What is Lupin's response? What do Ron and Hermione think, and are they correct?
Further Study 
- Why would Crookshanks only attack Scabbers and not other pets?
- Hogwarts' security is supposed to be impenetrable. How could Sirius Black have bypassed the safeguards?
- The Dementors work for the Ministry of Magic and are stationed at Hogwarts to protect Harry while searching for Sirius Black. Why would Harry fear them more than Voldemort or even Sirius Black? Based on what has been seen, is Harry's fear justified?
- Why would Snape give Lupin a potion, and what might it be?
- Why does Lupin seem unhappy during the feast, and why is Snape watching him so intently?
Greater Picture 
No one knows how Black broke into Hogwarts, though it is clear that he has somehow entered the castle grounds. We learn later that there are two secret passages into the school, one starting at the Shrieking Shack and ending at the Whomping Willow, and another that goes into the school's interior from the cellar in Honeyduke's Sweetshop in Hogsmeade village. Possibly Black broke into the Shrieking Shack and used the tunnel from there, or sneaked into Honeyduke's cellar. He might also have simply have walked in; the protective spells around Hogwarts must be specific to humans, as owls, for instance, can pass freely in and out of the school grounds. However, we cannot know whether these spells can detect Black in his Animagus-transformed canine shape. Black explains later that the Dementors perceived him as a dog when he was transformed, though Harry's Marauder's Map, which he will receive just before Christmas, will detect another Animagus' human identity while he is transformed. That Animagus is later revealed to be Peter Pettigrew, who is believed to have been murdered by Sirius.
Crookshanks definitely does know the difference. He never attacks Neville's toad, Trevor, or Pigwidgeon, the tiny owl Black later gives Ron, but instead targets only Scabbers, even though all are creatures that Crookshanks would normally hunt. His attacks seem motivated by what Scabbers actually is: an Animagus. Black, also an Animagus, explains later that it took a long time for him to earn Crookshanks' trust and procure his help.
It will be some months yet before Harry learns that Black is widely believed to have betrayed James and Lily Potter to Voldemort, and some months after that before he learns the truth. Until he hears the generally-accepted story, he, and we, are unable to determine why the authority figures are reluctant to allow him to visit Hogsmeade. Black apparently wants to murder Harry, though Harry sees this threat as little more than what he previously faced three times from Voldemort. With the Dementors prowling Hogsmeade and protecting the school, the authorities likely feel Harry was safe from Black, but are uncertain whether Harry knows the commonly-believed story that Black betrayed Harry's parents. Likely they assume that if Harry did know, he would attempt to find Black, thus removing him from the Dementors' protection.
It may be significant that the authorities, with the possible exception of Dumbledore, have failed to consider that if Black did slip past the Azkaban Dementors unnoticed, he could probably do so again to reach Harry inside Hogwarts. Also, no one, excepting Lupin, has noticed the Dementors' strange attraction to Harry, perhaps making them more dangerous to him than Black appears to be.
Readers, like Harry, may suspect that Snape has brewed Lupin a tainted drink. It is, however, another clue regarding Lupin's, as yet, unknown condition. Snape has given him a Wolfsbane potion that allows Lupin, who is a Werewolf, to retain his sanity during his monthly transformations. Without the potion, he becomes a savage, uncontrollable killer. Lupin will later mention that he had no fear of being poisoned, as it would have been much easier for Snape to simply make the potion incorrectly; one incorrectly mixed dose, and Lupin would probably injure a student and be dismissed or imprisoned. And Wolfsbane Potion is extremely complicated to make, so Snape would be able to claim, successfully, that it was a simple mistake on his part.
It is a little curious that Snape is so intently watching Lupin at the Hallowe'en feast. Of course, Snape is concerned that he has created the Wolfsbane potion correctly, but as it is now the end of October, there must have been one or two full moons during the school year preceding the events in the chapter, so there must have been at least one previous use of the potion. We must assume that Snape's concern also occurred on those occasions, but that the Trio did not note it because they were as yet unaware of the use of the potion.
Just as a side note, we should mention that it was very risky for Lupin to be on the Hogwarts Express. If a full moon occurred near 31 October, the previous one would have fallen near 2 October, and the one before that on 3 or 4 September. As the Hogwarts Express travels on 1 September, Lupin's journey would have fallen squarely into the one-week period preceding the full moon during which he must take the potion every day. It would have been extremely easy for Lupin to have missed a dose because he was in transit.
It is mentioned that Lavender and Parvati spend much time with Professor Trelawney. Their near-obsession with Divination extends beyond this one book; they apparently pass their Divination OWL exams, because in their first year of NEWT-level studies, in Harry Potter and the Half-Blood Prince, both go off to Divination.
Though it is relatively minor plot point, it is worth noting here that Professor Lupin shows Harry the new Grindylow when he invites him to his office for tea. Several years later (in book 7), after Death Eaters attack Order of the Phoenix members who are transporting Harry to a safe house, Lupin, to confirm Harry's identity, asks him what creature was in his office the first time Harry visited.
- We see Lupin's office, and Harry notes the difference between Lupin's chosen decor and Professor Lockhart's. Harry will note similar differences in style when visiting this same office when it is occupied by Professor Moody and Professor Umbridge.
- The Grindylow that we see in Lupin's office will be mentioned again in Harry Potter and the Deathly Hallows, when Lupin needs to confirm Harry's identity. We will also see Grindylows in the Second Task of the Triwizard Tournament, in Harry Potter and the Goblet of Fire; Harry will not, however, use the knowledge of the Grindylow's fragile fingers that Lupin here passes to him, to escape their clutches.