Over the Easter holiday, Harry lies to Hermione that he no longer has Occlumency lessons because Snape thinks he's doing well. Harry pretends to study and dwells on what he saw in the Pensieve. Hagrid and Sirius have always told Harry that James was wonderful, but he can't imagine ever tormenting someone like James tormented Snape. He's also disturbed by Lily's clear hatred of James in the memory.
Now that Harry knows that James wasn't without faults, he can begin to decide how much like his father he'd like to be. Notably, Harry has this choice; he gets to actively choose what parts of his father to attempt to embody. He doesn’t yet fully realize that he has this choice, however, and so feels crushed to have one of his idols destroyed in this way.
At the end of the holiday, Ginny finds Harry in the library to give him Mrs. Weasley's Easter package of chocolate eggs. The gift inexplicably makes a lump rise in Harry's throat and Ginny asks if Harry's okay. He says he'd like to talk to Sirius. Ginny says that they can figure something out, but then the librarian throws them out of the library. Back in the common room, Harry finds a notice listing appointments with McGonagall so she can give career counseling, along with pamphlets discussing possible jobs. Several jobs require high N.E.W.T. scores in a number of subjects.
Though the novel never says so outright, students must achieve certain scores in their O.W.L.s in order to progress to the N.E.W.T. level. This means that all of the jobs that require high scores in Defense Against the Dark Arts will be nearly unattainable for non-D.A. fifth years, as they haven't had the training to be able to do well enough on the tests to earn high scores.
Fred and George interrupt Harry, Ron, and Hermione's perusal of the pamphlets to talk about arranging for Harry to talk to Sirius. George explains that once the break is over, they're going to start causing trouble and can create a diversion so Harry can use Umbridge's fireplace to talk to Sirius. Hermione is distraught, but Ron says that Harry can make his own choices. Fred promises Harry 20 minutes, and tells Ron where to find the diversion. The next day, Hermione spends every minute giving Harry dire warnings. Snape ignores Harry all through Potions, which Harry thinks is an improvement. When he takes a flask to Snape for grading, however, Snape "accidentally" drops Harry's potion and Harry discovers that Hermione already Vanished what was left in his cauldron.
Ron's assertion that Harry can choose to speak to Sirius or not is one way that he can continue to show Harry his loyalty. While Hermione pesters Harry constantly about his decision—trying to keep him safe and counter Harry’s impulsive nature—Ron treats Harry with surprising maturity. What Harry saw in the Pensieve makes Snape’s hatred of James suddenly seem understandable, though it doesn’t justify Snape’s mistreatment of Harry, who shouldn’t have to pay for his father’s sins.
Harry arrives late for his career appointment with McGonagall and finds Umbridge in the office as well. At McGonagall's prodding, Harry says he'd like to be an Auror. McGonagall tells Harry what he needs in terms of O.W.L.s and N.E.W.T.s, including continuing with Potions, and ignores Umbridge's coughs. Umbridge finally interrupts and says that Harry doesn't have the temperament or good enough grades in Defense Against the Dark Arts. McGonagall says that despite Harry's poor marks from Umbridge, under "competent" professors he's done well. Umbridge scribbles furiously and says that with Harry's criminal record, he can't be an Auror. Growing angry, McGonagall says that she'll coach Harry personally to help him achieve the required results.
McGonagall shows here that when push comes to shove, she'll advocate for her students and help them achieve their goals, no matter who's in charge of the school. This behavior from McGonagall, combined with her earlier warnings to lay low and not be noticed, suggests that this is something that's extremely important to her—and something she's willing to spend all of her capital as a teacher to fight for. She lets her dislike of and disdain for Umbridge show here, but not to the point that she lets herself get fired—it’s too important for her to stay and protect her students.
In Defense Against the Dark Arts later, Harry guiltily thinks of what McGonagall would say if she were to find out that he trespassed in Umbridge's office after vouching for him. Harry thinks of Sirius saying that he's not as much like James as Sirius thought, and wonders if he even wants to be like his father. As the bell rings, Harry hears a bang and he runs toward Umbridge's office. He slips on his Invisibility Cloak and uses a special knife from Sirius to unlock the door. He uses Floo powder to send his head to Grimmauld Place and finds Lupin in the kitchen. Lupin runs to fetch Sirius.
Harry's musings about whether he even wants to be like his father suggest that he's accepting that he does have a say over what kind of person he'll be in the future, no matter what his father was like. Having some context from Sirius and Lupin, however, will allow Harry to gather more information and make a better choice in regard to how he thinks of his father, and how he thinks of himself as his father's son.
Harry tells them about what he saw in the Pensieve. Sirius and Lupin explain that Snape was jealous of James, but James was an arrogant jerk at age fifteen. Lupin admits that he never told Sirius and James to leave Snape alone, so it's his fault too. Sirius explains that James always showed off around Lily and that she didn't really hate him. They started dating in seventh year, after James stopped being so arrogant. Sirius tells Harry that James grew out of being an idiot. Then he asks how Snape reacted to Harry seeing his memory. They're concerned when Harry says Snape won't teach him Occlumency anymore, and tell Harry to go speak to Snape. Then they hear footsteps and Harry pulls his head out of the fire. He throws on the Cloak just as Filch enters and then follows Filch out of the office.
Though Harry isn't able to get much important information over the course of the novel, understanding the kind of teenager James Potter was allows Harry to feel more secure and okay with the person his father was. In a larger sense, this scene also shows Rowling continue to complicate Harry’s world as he grows up, with the novels themselves maturing in a sense along with their protagonist. In previous books James was presented as a saint-like hero, but now he is revealed to be just another human with very human flaws.
Harry pulls the Cloak off when he's far enough away and finds students covered in a stinky, gooey substance with Fred and George surrounded in the entrance hall. When Umbridge says that she's going to punish the twins, the twins Summon their broomsticks. They announce that students can buy a Portable Swamp like the one upstairs at Weasleys' Wizard Wheezes in Diagon Alley. They tell Peeves to torment Umbridge and then fly away, as all the students applaud.
Umbridge's anger implies that the Portable Swamp is no small or inconsequential thing, which again reminds the reader that the twins are smart and competent wizards, even without their test scores. Further, the fact that they must leave school after setting up the swamp shows that these grander acts of rebellion aren't sustainable in the long run, though this is still a satisfying and triumphant moment for the twins and for all those who hate Umbridge.