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It's almost impossible for there to be a completely faithful book-to-movie adaptation. Budgetary constraints, directorial choices and the limitations of visual media mean changes are inevitable. For a book series set in the expansive Wizarding World, it was a given that the Harry Potter movies were not going to be a direct copy of the books as soon as they were announced. Harry Potter and the Sorcerer's Stone changed several aspects of the book it adapted. Here are a few of the biggest changes.
The Dursleys' Introduction as Harry Potter's Adoptive Family
The Harry Potter series kicked off with a scene every movie fan can instantly recognize: Harry clambering out of his cupboard while being harassed by his adoptive family. It perfectly explained the dynamic between Harry and the Dursleys in very little time. While the book had a similar interaction, it took a more dedicated approach to Harry's extended family. The very first chapter of Harry Potter and the Sorcerer's Stone illustrated a day in Vernon Dursley's painfully ordinary life. For the first and only time, the reader visited his firm, Grunnings, and returned to Aunt Petunia at Privet Drive, who had an equally mundane day, ignoring Dudley's antics.
As unpopular as the Dursleys are, the first chapter did an excellent job of introducing them. Its purpose was solely to show that these people were meant to be as different from Harry and the rest of the magical community as possible, and it did so succinctly. As the series progressed, the Dursleys continued to prove that it was indeed an accurate assessment of their family. None of them received any worthwhile character development, save for Dudley, who delivered a half-hearted apology to Harry near the very end.
Harry's Visit to Diagon Alley Was More Exciting in the Book
The movie iteration of Harry's first visit to the wizarding marketplace retained most of the elements the book described, but, as always, a direct page-to-screen adaptation was impossible. He visited Diagon Alley with Hagrid after being rescued from the Dursleys. In the Harry Potter and the Sorcerer's Stone movie, Harry got his school supplies without much incident, but he had a marginally more exciting day in the book.
While visiting Madam Malkins, Harry encountered an unpleasant young wizard who he'd soon come to know as Draco Malfoy. Their first meeting went about as well as one would expect, but Draco's appearance wasn't the only change that the movie made to the Diagon Alley trip. Right after Harry was done getting his school supplies, he was escorted to King's Cross Station to catch the Hogwarts Express by Hagrid. In the books, however, he spent one final month with the Dursleys perusing his new textbooks before setting off for Hogwarts.
How Harry Potter and His Friends Met Fluffy
While Harry and Ron were always up for a bit of rule-breaking in their first year, Hermione most definitely was not. Their personalities were quite similar in both versions of the story, but the Harry Potter and the Sorcerer's Stone book made it a point to highlight how different they were on the night they encountered Fluffy. The movie brought them face to face with the monster quite by accident when a staircase they were climbing maneuvered the trio into the forbidden corridor, but they played a more active role in the adventure in the book.
Both Harry and Ron were challenged to a midnight duel by Draco. When Hermione realized they intended to accept it, she made it her task to prevent them from attending the duel, an endeavor that trapped her outside the Gryffindor common room with her two companions. They arrived at the agreed-upon meeting place to find that Draco had tricked them and instead alerted Argus Filch to the possibility of rule breakers that night. They ran into Fluffy's room while trying to escape from Filch, which was quite similar to the movie version of Harry Potter and the Sorcerer's Stone, but the circumstances surrounding their escapes were different.
The Nimbus 2000's delivery
As a rule, first years at Hogwarts aren't allowed their own broomsticks or to play Quidditch. However, due to another run-in with Draco Malfoy, and some intervention from Professor McGonagall, Harry found himself the Seeker of the Gryffindor House Quidditch team.
Securing a place on the team meant that Harry needed a broom of his own. Soon after his recruitment to the team, one arrived by owl package for him. In the Harry Potter and the Sorcerer's Stone movie, Harry and Ron wasted no time ripping into the packaging to reveal Harry's new Nimbus 2000 at the Gryffindor table, much to the amazement of their surrounding housemates. Conversely, in the books, Harry was warned by a stern note attached to the package not to open it at breakfast. He and Ron had to wait till they were in the privacy of their common room to feast their eyes on the superb broomstick.
Peeves the Poltergeist and Hogwarts' Other Ghosts
Movie-only fans of the Harry Potter franchise probably won't be familiar with Hogwarts' resident poltergeist, Peeves. He's a recurring, albeit minor, character in the books whose only purpose is to cause as much mayhem to the staff and students of Hogwarts. He doesn't appear at all in the movie franchise, but he's quite a thorn in Harry's side in the books. Several of Harry's nighttime expeditions are complicated by Peeves' presence, and he relishes any opportunity to ruin a Hogwarts student's day.
Other ghosts, such as Nearly Headless Nick and the Bloody Baron, do appear in the movies, but their personalities are a far cry from how they're portrayed in the books. They're treated uncomfortably similarly to sideshow attractions in the movies instead of the autonomous entities with distinct personalities the books described them as.
The Hunt for the Sorcerer's Stone
Harry, Ron and Hermione had long suspected Severus Snape of trying to steal the Sorcerer's Stone. As soon as Dumbledore left Hogwarts, they suspected the worst and rushed back to the forbidden corridor to confront their teacher. Before they could get face to face with the thief, they had to get past unique traps set by several Hogwarts teachers.
Both the Harry Potter and the Sorcerer's Stone movie and book featured Fluffy the Three-Headed Dog, a patch of Devil's Snare, a roomful of enchanted keys and Professor McGonagall's giant chess set, but the movie left out two more traps: Professor Snape's riddle of potions and a guard troll left by Professor Quirrell. Snape's trap consisted of an enclosed room with two exits barred by flames, a row of potions and a riddle. It was no problem for Hermione, who quickly figured out the correct bottles she and Harry needed. As for Quirrell's troll, it wasn't a problem at all. The trio's adversary had taken care of it long before they arrived.