It's never possible to include every single moment in a book in its film adaptation, especially when the book in question contains the richly developed world of Harry Potter and its many intricate characters. That's why it's easy to forgive movie makers for not including the many details that makeup Neville Longbottom's development from awkward pre-teen to full-fledged hero.
That said, some of Neville's moments were changed so vastly between the books and films that it rendered the young man almost an entirely different character. Many of his missing moments from the books would have really enriched the films, too.
Visiting His Parents At St. Mungo's
One of the most heartbreaking scenes in the entire Harry Potter series is the one in which Neville goes to visit his parents, the Aurors Frank and Alice Longbottom. Driven mad after being tortured by Bellatrix Lestrange, the couple do not recognize their son and are incapable of speech. Neville seems embarrassed by them, which infuriates his austere grandmother, but he keeps the little candy wrappers that his mother gives him as gifts.
It's a scene fans can barely read without tears, and it would've been quite moving to see. In some ways, it's worse than having his parents gone and gives him more in common with Harry.
Standing With Harry
At one point during the battle in the Department of Mysteries during Harry Potter and the Order of the Phoenix, Harry is alone, being taunted by the Death Eaters that there's no one to help him. Neville, complete with his broken nose and blood gushing down his face, bravely announces, "He's not alone. He's still got me!" His words are even garbled due to his injuries, but he doesn't care and proves himself the most Gryffindor he's ever been at this moment.
While we get to see Neville act bravely in the films, this truly is one of his most heroic scenes, and it's too bad it didn't make the cut.
He Gave The Death Eaters At Hogwarts Hell
When Harry, Ron, and Hermione were out searching for Horcruxes, Neville, Ginny, and the rest of the Hogwarts students were fighting for their lives against the Death Eaters running their school. Despite the torture that awaited them, Neville and the D.A. still gave the Death Eaters hell, defying them at every turn and making it difficult for them to run Hogwarts.
A battered and bruised Neville meets Harry, Hermione, and Ron not as a bullied little boy but as a young man who actually sees more violence on a daily basis than the trio at this point, leading many fans to desire a film about this year at Hogwarts.
He Led Dumbledore's Army In Harry's Absence
Neville may have begun as a meek sort of character, but by the time the final book was printed, he was the ultimate badass. He led the D.A. in the Room of Requirement and safeguarding students while simultaneously leading the resistance against the Death Eaters occupying Hogwarts. He did it at a much more brutal time than Harry did it, and he did it without the support of Ron and Hermione--although students like Ginny helped as well.
Neville's formidable gran, who always judged the boy rather harshly, took great pride in her grandson after the courage he displayed that year.
He Was Asked To Join Slug Club
Why wouldn't Neville be invited to Slug Club? Horace Slughorn collected students with famous histories and parents, and his parents were known to be two of the bravest Aurors who ever lived. He was invited to attend the first meeting along with Harry; both were summoned on the way to Hogwarts during Harry Potter and the Half-Blood Prince.
Reducing Neville to a waiter was a silly choice, especially since he wasn't allowed back in the club when Slughorn learned of his lack of magical prowess. Little did he know just how much Neville had in him. Once Neville became a teacher at Hogwarts, perhaps Horace had more respect for him.
Voldemort Wants Neville On His Team
When Neville trudges forward to meet Voldemort in The Deathly Hallows, it's a sham of the scene in the book, where he bravely runs at the villain. Voldemort actually laughs at Neville in the movie, mocking him, saying he was hoping for better!
This is the most outrageous offense against Neville, as he looked quite mighty at this moment. Voldemort definitely wanted to recruit him, admiring his spirit and bravery, and saying, "You come of noble stock. You will make a very valuable Death Eater. We need your kind, Neville Longbottom." It's a completely different scene from the one we see in the film, and it's a total disservice to this character.
He's Not The Butt Of So Many Jokes
While it's true that Neville gets bullied by students, as well as by the cold and cruel Professor Snape, he's not a complete idiot, and he's certainly not meant to be laughed at by the audience. In the films, he's often used as the butt of the joke, providing comic relief where there isn't any, and it's just lazy storytelling. The story shouldn't make us laugh at a character like Neville, who is authentic and kind, and who could very well have been the Chosen One himself.
Indeed, Neville acts like the Chosen One without the fame and appreciation that comes with the role, and we miss that in the movies.
He Could Have Been The Chosen One
Readers of the books know that when Professor Trelawney delivered her prophecy about the Chosen One, the Dark Lord's actions actually sealed Harry's fate. Neville Longbottom fit the same description as the prophecy, which meant that had Voldemort chosen to seek him out to destroy instead, the series would have been very different, and Neville may have been the one Draco dubbed "Scarhead."
Fans who've only seen the films have no idea about this since it's never even mentioned. Neville's importance is downplayed like this several times in the books, rendering his final stance as a dumb luck sort of moment rather than the culmination of his development.
A More Heroic Stance Overall
While Matthew Lewis did a great job and even blossomed alongside his character throughout the whole series, he doesn't get the chance to play the Apocalyptic fighter that Harry, Hermione, and Ron meet when they return to Hogwarts in the book. He still has moments of uncertainty and hesitation in the film, while book-Neville is incredibly sure of himself after becoming a seasoned resistance fighter.
He charges ahead like a warrior, a hardened soldier who has seen much since his days as a First Year. Believing Harry to be dead doesn't render him weary but enrages him instead. It's both heartbreaking and inspiring, and movie fans are sorely missing out by not witnessing his transformation.
The Scene With Nagini
While Neville did take out Nagini in The Deathly Hallows: Part 2, the scene lacked its book energy. When Neville believes Harry is dead, he charges at the Dark Lord. He is fierce. Voldemort not only has to put him in a Body-Bind, but he also sets the Sorting Hat on fire after forcing it on Neville's head! It is horrific, but it makes Neville's actions even more incredible afterward.
He breaks free of the Body-Bind on his own, in a moment of distraction. The hat produces the sword, and he doesn't even hesitate. When he decapitates Voldemort's beloved snake and Horcrux, it is with determination and confidence, despite his pain, and it would've been one of the most amazing moments in the series. And now that we've seen who Nagini really is in the Fantastic Beasts saga, it's harder to witness, too.