Professor Snape
Background
Severus Snape is the professor of potions at Hogwarts School of Witchcraft and Wizardry. He is the head of Slytherin House. Snape is a former member of the Death Eaters. In school, he signed his book as the Half Blood Prince.
During the events of Harry Potter and the Sorcerer's Stone (2001) played by Alan Rickman
During dinner, Harry Potter sees Snape staring at him. The next day, Snape introduces his class and points out that fame doesn't equal ability, after testing Harry's knowledge of potions, which Harry fails. Professor McGonagall, Snape and Professor Quirrell run into the bathroom and Hermione Granger takes the blame for not being in the common room and Gryffindor loses five points, but gains ten for Harry and Ron Weasley's bravery. Harry notices that Snape has a limp and thinks Snape was trying to get whatever Fluffy is guarding. During the Quidditch games, as Harry starts to go after the Snitch, his broom starts to act on its own. Hermione sees Snape mouthing spells and she starts a fire under Snape, who bumps into a professor who bumps into Professor Quirrell. Harry is able to regain control of his broom and catches the Snitch in his mouth and wins the game for Gryffindor. Harry uses the cloak to go into the restricted area of the library and is almost caught by Mr Filch. As he is trying to get away, Harry sees Snape confront Professor Quirrell about his loyalty. Mr Filch shows Snape and Professor Quirrell the lantern he found in the restricted section.
During the events of Harry Potter and the Chamber of Secrets (2002) played by Alan Rickman
Harry and Ron are brought to Professor Snape's office where Professor Snape tells them they were seen by seven people flying in the Ford Anglia and they damaged the Whomping Willow. He threatens to kick them out and Albus Dumbledore and Professor McGonagall walk in and Professor McGonagall gives Harry and Ron detention. After Mrs Norris is petriied, Dumbledore and the professors walk up and Dumbledore tells the students except for Harry, Ron and Hermione to return to their rooms. Professor Snape says it is suspicious how Harry was present at the time and Gilderoy Lockhart tells him, Harry was with him. Dumbledore then mentions Professor Sprout's Mandrakes and tells Mr Filch, Mrs Norris will be fine. After a student is petridied, Lockhart and Professor Snape introduce a Dueling Club so the students can practice defending themselves. Harry and Draco Malfoy are pitted against each other and during the match, Draco creates a magic snake. The snake starts to slither towards Justin Finch-Fletchley and it appears that Harry is commanding the snake with Parseltongue. Professor Snape destroys the snake with a spell and Justin Finch-Fletchley angrily thinks Harry was sending the snake after him. Later, Professor McGonagall tells Professor Snape, Madam Pomfrey, Professor Flitwick, and Mr Filch, Ginny Weasley was taken to the Chamber of Secrets and all the students must be sent home. Before the end of the year feast, Dumbledore asks for a round of applause for Professor Sprout and Madam Pomfrey and all of the students and professors clap, including Madam Pince. Hagrid returns and everyone gives Hagrid a round of applause.
During the events of Harry Potter and the Prisoner of Azkaban (2004) played by Alan Rickman
In Protection Against the Dark Arts class, Professor Snape substitutes for Professor Lupin and teaches the students about werewolves and Animagus. Snape catches Harry walking around the corridors in the dark, but not before Harry manages to change the Marauder's Map back to a simple parchment. He uses magic on the map and tells Harry to tell him what it says and Harry tells him the parchment said to keep his big nose out of other people's business. Lupin walks up and takes the map after telling Snape it is probably a parchment of insults against those who use magic against it. The Old Man in Portrait then yells at Snape to turn off his wand light. In the Shrieking Shake, Snape runs into a room and disarms Sirius Black. Harry then uses his wand to knock Snape unconscious. Snape regains consciousness and walks out of the tunnel and is swiped at by Lupin who has completely transformed into a werwolf. Sirius and Lupin fight and Harry throws a rock at Lupin distracting him. Something from far away howls and Lupin runs towards the howl.
During the events of Harry Potter and the Goblet of Fire (2005) played by Alan Rickman
During the champion selection for the Triwizard Tournament, Cedric Diggory, Viktor Krum, Fleur Delacour and Harry's names are chosen. Madame Olympe Maxime and Igor Karkaroff accuse Harry of cheating, but Barty Crouch says Harry must compete. McGonagall tells Dumbledore he can't allow Harry to compete, but Snape thinks they should allow Harry to compete until they figure out what is going on. After Harry passes the second challenge, Snape accuses Harry of stealing from his potion stock and threatens to use Veritaserum on him. After he takes Harry away, Alastor "Mad-Eye" Moody starts to transform and as he is about to kill Harry, Dumbledore bursts into the room and stuns Moody with magic. Snape pours Veritaserum into Moody's mouth. Dumbledore, Snape and McGonagall find the real Moody locked in a chest and the fake Moody fully transforms back to Barty Crouch Junior.
During the events of Harry Potter and the Order of the Phoenix (2007) played by Alan Rickman and Alec Hopkins
Sirius is having a meeting with Mr Weasley, Lupin, McGonagall, Mrs Weasley and Snape. The meeting ends and Harry says hello to everyone and Sirius gives him a hug. Kingsley Shacklebolt hands Harry a newspaper where Cornelius Fudge is accusing him and Dumbledore of lying about Voldemort. Sirius tells Harry that Voldemort has been trying to recruit followers and that Voldemort is trying to get something he didn't have last time. During the Hogwarts banquet, Dumbledore welcomes back Professor Grubbly-Plank as the Care of Magical Creatures while Hagrid is away. He then introduces Dolores Umbridge as the new Defense Against the Dark Arts teacher. Dolores interrupts Dumbledore and tells the students that the Ministry of Magic is very concerned about their education. Later, Snape takes Harry down to his office and trains him in the use of Occlumency. Snape continues to train Harry in Occlumency, and Harry manages to get into Snape's memories and sees a memory of Snape's where he is picked on by James Potter, Sirius, Peter Pettigrew and Remus. Snape kicks Harry out of his mind and refuses to train him in Occlumency anymore. Dolores tells Snape to bring her truth juice, but he tells her that she used the last of it on Cho Chang.
During the events of Harry Potter and the Half-Blood Prince (2009) played by Alan Rickman
Bellatrix Lestrange, Narcissa Malfoy, Peter, and Snape have a meeting and Snape offers to help Narcissa. Snape then creates an Unbreakable vow with Narcissa to help Draco when he fulfills Voldemort's wishes. Dumbledore introduces the students to Professor Horace Slughorn and tells the students that dark forces are trying to penetrate Hogwarts after telling them that Snape will be the new protection against the dark arts teacher. After Katie Bell is cursed, Harry finds a necklace and brings it to McGonagall who questions Leanne about it and Leanne tells her that Katie told her she had to deliver it to Dumbledore. Harry accuses Draco of cursing Katie to McGonagall and Snape.
At Slughorn's Christmas dinner, Draco is caught near the party and brought before Slughorn who is with Eldred Worple and Snape drags him outside and Harry overhears Snape offering to help Draco complete Voldemort's task. Later, Draco casts Expelliarmus on Dumbledore, disarming him. Bellatrix, Fenrir Greyback, Amycus, Alecto, and Rowle walk over to Draco and Snape sneaks up on Harry. Snape goes to where the others are and casts Avada Kedavra on Dumbledore, killing him. Snape leaves with the Death Eaters who, as they are leaving cause destruction within Hogwarts and burn down Hagrid's hut. Harry attacks Snape and Bellatrix stuns Harry with a spell. Snape reveals himself to be the Half-Blood Prince and walks away.