Rowling was less forthcoming about Snape than she was for other characters, because his true loyalties and motivations were not to be revealed until the final book. However, she hinted numerous times at Snape's important role, suggesting that people should "keep an eye on Snape." Answering a question regarding Snape's love life and the redemptive pattern to his character in 1999, Rowling expressed her surprise at the foresight.
After the completion of the series Rowling began to speak openly about Snape and admitted that she was particularly pleased with the way Snape’s story played throughout the course of the series, contrasting his character arc with that of Albus Dumbledore. Saying, "the series is built around (Dumbledore and Snape)", Rowling maintained that she always knew what Snape would turn out to be at the end and that she carefully plotted his storyline throughout the series, " I had to drop clues all the way through because as you know in the seventh book when you have the revelation scene where everything shifts and you realize why Snape was… what Snape’s motivation was. I had to plot that through the books because at the point where you see what was really going on, it would have been an absolute cheat on the reader at that point just to show a bunch of stuff you’ve never seen before." Rowling further said in an interview that she wanted redemption and forgiveness for Snape, "Snape is a complicated man... he's a very– he was a flawed human being, like all of us. Harry forgives him– as we know, from the epilogue, Harry– Harry really sees the good in Snape ultimately... there's redemption."
Snape has a minor role in Harry Potter and the Chamber of Secrets, where he helps Gilderoy Lockhart oversee Hogwarts short-lived Duelling Club but has little interaction with the main plot. It is while attending the Duelling Club that Harry learns the Expelliarmus spell indirectly from Snape, a spell that plays a significant role in later books. In Harry Potter and the Prisoner of Azkaban, Snape demonstrates his expertise with potions by brewing the complex Wolfsbane potion for the new Defence Against the Dark Arts professor, Remus Lupin. Snape harbours a deep suspicion throughout the third book that Lupin may be assisting Sirius Black to enter Hogwarts Castle, believing that Black intends to kill Harry. This suspicion stems from Lupin's friendship with Sirius and Harry's father James while they were all at Hogwarts as students. Near the climax of the book, Snape attempts to apprehend Black, believing Black is responsible for murdering innocent bystanders and betraying the Potter family's hiding place to Voldemort. When Black escapes, Snape rightly accuses Harry of aiding him, still believing that Black is a mass murderer. After Harry and Lupin escape punishment, Snape retaliates by revealing to the entire school that Lupin is a werewolf, forcing the latter to resign his post.
Prisoner of Azkaban reveals more details about the connection between Snape and James Potter. While in school together, Sirius once tricked Snape into entering the Shrieking Shack while Lupin was there, transformed into a werewolf. James realised the danger and stopped Snape, saving his life; this is the incident Dumbledore referred to at the end of the first book. Snape, however, believes James's actions were self-serving, to avoid being expelled.
At the end of the book, Dumbledore attempts to convince a disbelieving Minister for Magic, Cornelius Fudge, that Voldemort has returned. As proof, Snape willingly shows Fudge the restored Dark Mark on his arm, and is subsequently sent on a secret mission by Dumbledore. This mission, as implied in Harry Potter and the Order of the Phoenix and revealed in Harry Potter and the Half-Blood Prince, was to rejoin the Death Eaters and spy on Voldemort as a triple agent, pretending to spy on Dumbledore on behalf of Voldemort.
In the fifth novel, Harry Potter and the Order of the Phoenix, Snape returns to a more prominent role. With Voldemort having returned to a fully corporeal body, Snape continues working as a triple agent for Dumbledore. He is seen prior to the start of school at Number 12, Grimmauld Place giving reports to the Order of the Phoenix. He has a very strained relation with Sirius Black, who owns Grimmauld Place and must remain there in hiding. The two trade frequent snide remarks and at one point almost begin a duel. Snape taunts Sirius about the latter not being able to take an active role in the Order's missions due to his fugitive status. Harry later feels that this contributed to Sirius' willingness to take unsafe risks. Back at school, Snape's official allegiance to the Order has no effect on his dislike for Harry.
Later in the book, Dumbledore has Snape teach Harry Occlumency, the protection of the mind from outside intrusion or influence. Snape is an expert in Occlumency. The sessions are made difficult by their mutual hostility and end prematurely when Harry uses Dumbledore's Pensieve to view, without Snape's permission, a childhood memory of Snape being bullied by James Potter and Sirius Black, and of him insulting Lily Evans, Harry's mother. Neither Harry nor the reader discovers until the final book that this confrontation marked the end of Snape's friendship with Lily.
Towards the end of the novel Dolores Umbridge captures Harry and questions him on the whereabouts of Dumbledore. She sends for Snape to provide a truth serum to force Harry to reveal any information he may be hiding. Snape claims that his supplies of Veritaserum are exhausted, when she desperately attempts to use the drug surreptitiously to force information from Harry. Snape withholds further assistance. It is later revealed that Snape had in fact supplied Umbridge with fake Veritaserum on a prior attempt. Snape then carries Harry's cryptic warning about Sirius's capture to the other Order members, allowing them to come to the rescue in the Department of Mysteries. Harry, however, still holds Snape partly responsible for Sirius' death, believing Snape's goading spurred Sirius into joining the battle.
At the Start-of-term feast at Hogwarts, Dumbledore announces he has finally appointed Snape as Professor of Defence Against the Dark Arts. Horace Slughorn, a retired Hogwarts teacher, replaces Snape as Potions Master. Slughorn lends Harry an old Potions textbook, in which Harry finds marginalia including a variety of hexes and jinxes seemingly invented by an unknown student, and substantial improvements to the book's standard potion-making instructions. The book is inscribed This Book is the Property of the Half-Blood Prince. The notes greatly bolster Harry's performance in Potions and he impresses Slughorn. Snape, who maintains that he "never had the impression that [he] had been able to teach Potter anything at all", is suspicious of Harry's newfound Potions success.
Later, in a fight with Draco Malfoy, Harry casts one of the Prince's spells (Sectumsempra) marked "For Enemies" and is horrified when it causes devastating wounds to Draco's face and chest. Snape rushes to the scene and heals Draco's wounds, and then interrogates Harry about the source of the spell, using Legilimency to extract the source of Harry's knowledge (the Potions textbook) from Harry's mind. As Harry refuses to hand over the Half-Blood Prince's book, Snape puts him in detention during the final Quidditch match of the year.
Returning to Hogwarts after a search for one of Voldemort's Horcruxes, Harry and Dumbledore alight on the school's astronomy tower. Gravely weakened by Voldemort's protective potion, Dumbledore asks Harry to fetch Snape. Before Harry can leave, Draco Malfoy suddenly arrives intending to carry out Voldemort's ordered assassination of Dumbledore, but cannot bring himself to commit the murder. The arrival of Death Eaters and Snape interrupt them, and Snape kills the headmaster himself. An enraged Harry (who had been paralysed by Dumbledore and witnessed the killing while under his invisibility cloak) chases Snape, Malfoy, and the Death Eaters as they flee the castle. Snape easily blocks Harry's attempts to attack him with magic and even jeeringly points out Harry's mistakes, but refuses to strike back. During the confrontation, Snape reveals himself to be the "Half-Blood Prince" (being the son of Muggle Tobias Snape and pure-blood Eileen Prince). Harry is unable to stop Snape before the latter passes through the school gates and Disapparates. The full story of the relationship between Dumbledore and Snape and the real reason for the killing are not revealed until the next and last book. Rowling mentioned in an interview that at this point in the series the relationship Harry-Snape has become "as personal, if not more so, than Harry-Voldemort.
Towards the end of the school year, Professors McGonagall, Flitwick, and Sprout force Snape to flee the school. Later Snape is summoned to the Shrieking Shack by Voldemort. Erroneously believing Snape is the master of the Elder Wand, Voldemort betrays Snape and has his pet snake Nagini bite him through the neck and mortally wound him, believing that Snape's death would make him the master of the Wand. Snape, dying from his wounds, releases a cloud of memories and tells Harry to take them. From these memories, Harry sees Snape's childhood and learns of his true loyalties. Harry observes that Snape befriended Lily Evans, Harry's mother, as a child when they lived near to each other. Upon their arrival at Hogwarts, the Sorting Hat placed Snape and Lily into Slytherin and Gryffindor Houses, respectively. They remained friends for the next few years until they were driven apart by Snape's interest in the Dark Arts; the friendship finally ended following the bullying episode that Harry had briefly seen in the fifth book. Despite this separation and Snape's loathing of Lily's eventual husband James Potter, who had bullied Snape at Hogwarts, Snape remained in love with Lily. When Snape inadvertently revealed the prophecy made by Sybill Trelawney to Voldemort, Voldemort decided to attack the Potters in an attempt to prevent its fulfilment. Though he asked Voldemort to spare Lily, Snape, still fearing for her safety, went to Dumbledore and begged him to protect the Potters. Dumbledore agreed and ensured that they were placed under the Fidelius Charm.
In return, Snape became a double agent for the Order of the Phoenix against Voldemort, using his powers of Occlumency to hide his betrayal from Voldemort. Even with his efforts to protect her, Snape felt responsible for Lily's death when the Fidelius charm was broken. Despite Harry's strong resemblance to James Potter, the fact that he was Lily's son made Snape protect him throughout the series. Snape demanded of Dumbledore, however, that his love for Lily, his reason for switching sides, be kept a secret. Dumbledore agreed and kept the secret throughout the series.
Snape's memories then reveal that Dumbledore had been afflicted by a powerful curse cast on the Peverell ring, one of Voldemort's Horcruxes, prior to the start of Harry's sixth year at Hogwarts. Although Snape's knowledge of the Dark Arts enabled him to slow the spread of the curse, the curse would have ultimately killed Dumbledore within a year. Dumbledore, aware that Voldemort had ordered Draco Malfoy to kill him, asked Snape to kill him instead as a way of sparing the boy's soul and of preventing his otherwise slow, painful death. Although Snape was reluctant, even asking about the impact of such an action on his own soul, Dumbledore implied that this kind of mercy-killing or killing "by request" would not damage a man's soul in the same way murder would. Snape agreed to do as the Headmaster requested. Snape's memories also provide Harry with the information he needs to ensure Voldemort's final defeat, in the form of conversations Snape had with Dumbledore.
Rowling noted in an online interview that because Snape abandoned his post before dying or officially retiring, a portrait of him does not immediately appear in the Headmaster's office following his death. She adds, however, that she would like to think Harry made Snape's true loyalty and heroism known in the Wizarding world, and that he lobbied to ensure that a portrait be installed in the office. In a separate interview, Rowling discussed Snape's backstory, saying she had planned it ever since she wrote the first book because the whole series is built around it and she considers him one of the most important characters of the seventh book.
As of 2007 Severus Snape has appeared in all five Harry Potter films, portrayed by British actor Alan Rickman. Rickman was Rowling's personal choice to portray the character. He had conversations with Rowling about his character and is one of the few Harry Potter actors that she spoke to prior to the completion of the book series about the future direction of the character. "He knew very early on that he'd been in love with Lily. He needed to understand […] where this bitterness towards this boy who's the living example of her preference for another man came from."
Rickman himself has mostly refrained from talking about Snape, asking the readers to wait and “see what unfolds” in the course of the novels; however, he did say Snape is a complicated person, very rigid and full of himself; in an interview he went further, saying: "Snape isn't one who enjoys jokes and I strongly fear that his sense of humour is extremely limited... But in his defence, I will add that he didn't have an easy adolescence, particularly during his studies at Hogwarts." He also said Snape is a fascinating character, and that he takes immense pleasure in playing someone so ambiguous.
Rickman's performance as Severus Snape is popular with viewers and is appreciated among critics. Entertainment Weekly listed Rickman as one of the most popular movie stars in 2007 for his performance as Severus Snape, saying: "As the icy, humorless magic instructor Severus Snape, Rickman may not be on screen long — but he owns every minute. Rickman also noted the fans' reaction, in an interview he said that he finds it splendid that people in general adore Snape.
In the chapter illustrations by Mary GrandPré in the American edition of Harry Potter and the Order of the Phoenix, Snape is depicted as balding with a goatee, but in the next novel, Half-Blood Prince, he is depicted with long black hair.
The adult Snape, on the other hand, is shown very self-assured and confident of his abilities to a degree that Rickman described as ”full of himself.” Director David Yates said Snape is a character with gravitas, authority and power. Snape typically displays a very calm and collected demeanour and is in control, rarely at a loss for words or taken off guard. However his temper is sometimes short where Harry is concerned and positively flares dealing with his erstwhile tormentor Sirius and when accused of cowardice. His otherwise impassive and aloof attitude seems to stem from his belief that people who can't control their emotions are weak.
Like some other prominent members of Slytherin house, Snape is shown a clever and cunning wizard. He is intelligent and has a keen analytical mind. Rowling in an interview adds that Snape is immensely brave, and when asked if she considers Snape a hero, replied: "Yes, I do; though a very flawed hero. An anti-hero, perhaps. He is not a particularly likeable man in many ways. He remains rather cruel, a bully, riddled with bitterness and insecurity — and yet he loved, and showed loyalty to that love and, ultimately, laid down his life because of it. That's pretty heroic!"
After Harry Potter and the Half-Blood Prince, Snape's loyalty was a matter of intense debate among the fans. The issue was given special attention in the marketing campaigns of the last book Harry Potter and the Deathly Hallows. "Is Snape Good or Evil?" was one of the questions in Scholastic Inc. seven-question series, part of its marketing campaign of the book. As part of the Waldenbooks marketing campaign, two free stickers, one that said "Trust Snape" and another that stated "Snape Is A Very Bad Man" were available with the book. Borders Group published a separate book on the topic, “The Great Snape Debate,” containing essays and arguments from both sides of the debate.
Jenny Sawyer from The Christian Science Monitor commented on the character's development in the series. She claims that Snape is the only protagonist who genuinely had a choice to make and struggled to do the right thing, hence the only one to face a "compelling inner crisis". She believes the popularity of the character is due to the moral journey and inner conflict Snape undergoes within the series, as it is the hero's struggle and costly redemption that really matters: "[Snape's] character ached for resolution. And it is precisely this need for resolution – Our desire to know the real Snape and to understand his choices – that makes him the most compelling character in the Potter epic."
The final revelation of Snape's loyalty in Harry Potter and the Deathly Hallows was viewed positively by fans and critics alike. Daniel Radcliffe, who portrays Harry Potter in the movie series, expressed his delight, saying he was pleased to see that his theory that Snape would end up being a sort of tragic hero came through. Elizabeth Hand from The Washington Post wrote:
The much-maligned loner Snape does not come onstage until the latter part of "Deathly Hallows," but when he does the book becomes his: Snape's fate, more than Voldemort's, perhaps more even than Harry's, is the most heartbreaking, surprising and satisfying of all of Rowling's achievements.