While each Harry Potter character brings something to the table, Alan Rickman's Severus Snape is pretty much the heart and soul of the franchise. He was Hogwarts' Potions master, Lily Evans' childhood friend, Dumbledore's ally, and Harry Potter's teacher.
Snape certainly wasn't the friendliest on the Hogwarts staff. He was neither prone to smiling, nor making small talk with his students. He was undoubtedly hostile towards Harry, and his mates, but time did reveal his softer side. Snape is known as the multi-faceted Harry Potter character with a great many impactful quotes to his credit. Furthermore, his sarcastic remarks and backhanded compliments have just as much, if not more impact on the viewers.
"There Will Be No Foolish Wand-Waving Or Silly Incantations In This Class." - Harry Potter and the Sorcerer's Stone
...declared Snape as he burst through the door of the Potions classroom.
While everyone eagerly awaited the highbrow Professor, he didn't think much of his students. He considered Potions a complex field of work, and that is precisely why he said many of his students simply did not have what it took to excel at the subject. Potions was both science and art to Snape, and he was certain only a talented few (Draco Malfoy, in his opinion) would be able to live up to his expectations. Snape didn't think much of James Potter's son, Harry, and didn't hesitate to bully him on his first day.
"Pity. Clearly, Fame Isn't Everything. Is It, Mr. Potter?" - Harry Potter and the Sorcerer's Stone
The Boy Who Lived needed no introduction. His parents were martyrs in the First Wizarding War, and he himself was destined for great things. He'd become a legend after surviving the attack at Godric's Hollow.
Earlier, Hagrid had mentioned that Harry had had his name down at Hogwarts ever since he was born. Everyone, including the Hogwarts staff, knew quite a deal about him. Snape, however, disparagingly called him "a Celebrity." When the young wizard failed to answer a few of the Potion master's questions, the latter judged him too quickly and remarked he was merely famous.
"Put Your Hand Down, You Silly Girl!" - Harry Potter and the Sorcerer's Stone
Hermione Granger who sat beside Harry in the Potions class knew answers to all three of Snape's questions. She knew what she'd brew if she added Powdered Root of Asphodel to an infusion of Wormwood. She also knew what a Bezoar was and where it was found and finally, she was aware of the difference between Monkshood and Wolfsbane.
Eager to answer all questions, Hermione raised her hand thrice, but she was denied a chance to speak because Snape derived pleasure from interrogating Harry. He didn't care if anyone, including the well-read Ms. Granger, knew the answers, he just enjoyed humiliating an eleven-year-old.
"Silence! I Assure You That Were You In Slytherin And Your Fate Rested With Me, The Both Of You Would Be On The Train Home Tonight!" - Harry Potter and the Chamber of Secrets
In The Chamber of Secrets, Ron Weasley tried to evade punishment for flying the enchanted Ford Anglia to Hogwarts. Snape was furious as he and Harry risked the exposure of the Wizarding World to Muggles.
The enchanted car was spotted by a minimum of seven Muggles, and what's worse, the boys crashed it into the valuable tree - the Whomping Willow. While Snape would have preferred to send the boys home, he was helpless because they weren't in Slytherin. Minerva McGonagall slapped the boys with detention for flouting the Decree for the Reasonable Restriction of Underage Wizardry, Snape's verbal assault had a much worse impact on the boys.
"Weasley's Wand Causes Devastation With The Simplest Spells. We'll Be Sending Potter To The Hospital Wing In A Matchbox." - Harry Potter and the Chamber of Secrets
Ron Weasley's wand is damaged after the enchanted Flying Ford Anglia collides with the Whomping Willow on Hogwarts' grounds. His broken wand is a source of embarrassment to him in McGonagall's Transfiguration class and at the Duelling Club, started by the flamboyant Defense Against the Dark Arts teacher, Gilderoy Lockhart.
Snape takes this subtle dig at Ron's nuisance of a wand (and at Harry) when Lockhart volunteers the pair to a duel. Though Ron is slightly embarrassed, deep down he knows his professor is a cent percent correct.
"Are You Incapable Of Restraining Yourself, Or Do You Take Pride In Being An Insufferable Know-It-All?" - Harry Potter and the Prisoner of Azkaban
In the third Harry Potter movie - The Prisoner of Azkaban, Hermione angers Snape when she speaks out of turn in his class. He takes five points from Gryffindor and belittles her contribution. Snape is substituting for Professor Remus Lupin, and naturally, Harry wants to know why their Defense Against the Dark Arts professor is absent.
Snape, who appears more irritable than usual, comes down on the Trio. Ron, who has just had Snape turn his book page turned to 394, is the only one who seems to agree with him calling Hermione a know-all. She's offended but keeps mum because cheeky behavior doesn't go unpunished with Snape.
"Loss Of Limb Will Not Excuse You." - Harry Potter and the Prisoner of Azkaban
In the same class, Snape punishes students for being unable to tell the difference between an Animagus and a werewolf. He demands the class submit an assignment meeting the word length of two rolls of parchment paper on werewolves, with a special focus on how to recognize them.
Harry, who's got Quidditch the next day, has a perfectly understandable late assignment excuse, but when has Snape shown his students' leniency? Deadlines are important to him, and their violation is inexcusable.
"Vengeance Is Sweet. How I Hoped I'd Be The One To Catch You." - Harry Potter and the Prisoner of Azkaban
One of the most gripping scenes in the third Harry Potter movie - Encounter at the Shrieking Shack - is a chaotic altercation involving the Trio, three of the Marauders, Sirius Black, Remus Lupin, and Peter Pettigrew (disguised as his Animagus - pet rat Scabbers) and Professor Snape.
Sirius lures the Trio into the shack to get his hands on Peter Pettigrew, but Remus and Snape arrive one after the other. The latter two have some unfinished business with the man who sold Lily and James to Voldemort, but Snape, unaware of the whole truth, wants to nab the escapee, Sirius. The old foes argue endlessly, and Snape wants them out of the shack and into the castle. His words convey he's beyond thrilled to catch hold of the man wanted by the Dementors of Azkaban and the Ministry.