The first time Harry has potion class Snape asked him about asphodel and wormwood. Asphodel is a type of lily that means something like 'My regrets follow you to the grave' and wormwood symbolizes bitter sorrow. So you can combine both and they mean I bitterly regret Lily's death'.
Fred and George bewitched snowballs all the time that followed professor Quirrell and hit the back of his head. At the end of the book, we learn that Voldemort was living on back of Quirell's head; inside his turban. That means they were actually throwing snowballs on Voldemort's face.
The one and only time Fred and George saw each other as old men was in The Goblet of Fire. After Fred's death, George was never able to perform a patronus, and his name fell off from the Weasley clock they kept at home.
There's some symbolism through the story of the Peverell brothers. Snape for love, Voldemort died for power, and Harry could have escaped death, but in the end he revealed himself and greeted it gladly, as an old friend.
Some fans of the books, like myself, can't simply let go the fact that Harry's eyes weren't green in the movie. That was because the actor, Daniel Radcliffe, was allergic to the contact lenses
JK Rowling had only one requirement/rule over the casting decisions. She wanted all the actors to be British, since the students from Hogwarts come from England, Scotland, Wales and Ireland.
Fans of the books whish sometimes that few scenes were actually shown in the movies, but of course, they didn't make the cut because it's impossible to adapt such a huge book into a two hour length movie
In the books, the former professor of Defense against dark arts loses his memory due to a bad casted spell. In the movies, you can see that he never recovered, but he still signs autographs (there's a post credits scene in The Chamber of Secrets)
In Harry Potter and the Prisoner of Azkaban, there's a scene happening in the Leaky Cauldron, where we see a wizard reading a book. If you look close enough, you can see he's reading a Stephen Hawking's book
Dudley has a friend or wingman called Piers, who helps him to bully people, especially Harry, and is physically described as rat-like. Now, Piers is and old form of the name Peter
Professor Trelawney can be seen as a fake seer, but she is right most of the times. In the Prisoner of Azkaban, she says that when 13 people dine together, the first to rise shall be the first to die. In the fifth book, 13 Order members are dinning together, and the first to rise is Sirius
There's a scene in Harry Potter and the Goblet of Fire books where Dumbledore calmly asks Harry if he put the name into the Goblet of Fire. In the movie, Dumbledore is not calm at all.
There's a scene in The Deathly Hallows where Harry and Hermione are supposed to kiss. While shooting that scene, Rupert Grint had to be removed from the set. Why's that? Because he couldn't stop laughing.
When they were filming Harry Potter and the Prisoner of Azkaban, Tom Felton had his Hogwarts' pockets robes sown, because he was always trying to sneak food inside them
Who do you think is the eldest Hogwarts' student? Moaning Myrtle. The part was portrayed by Shirley Henderson, and she was born in 1965. That means she was 37 years old when Harry Potter and the Chamber of Secrets was filmed.
Sirius markings/tattoos aren't mentioned in the books. That was actually an idea from Alfonso Cuaron, the director of Harry Potter and the Prisoner of Azkaban. They can be roughly translated from alchemy symbols and ancient runes
Alfonso Cuaron had everyone write an essay about their characters, to get to know them. Emma Watson did a 16-page essay, Daniel Radcliffe wrote one page, and Rupert Grint never wrote it. You could say they remained true to their characters
There's a scene where Harry and Ron try to destroy over and over again Salazar's Slytherin locket. Forty versions of this prop were created in order to accommodate all the damages into the scene
In the 20th anniversary of Harry Potter and the Philosopher's Stone publishing, the New York Academy of Medicine created a sort of digital library where they showed rare objects that dated back to the 15th century, revealing the real story behind the saga elements, like mandrakes and basilisks
In Harry Potter and The Half Blood Prince, when Harry drinks Felix Felicis potion it helps him accomplish more than simply acquiring Slughorns' memory. It helps Ron to break up with Lavender and starts the final argument between Dean and Ginny
In Harry Potter and the Goblet of Fire, during a Divination class, professor Trelawney asks him if he was born in midwinter. Since Harry as born in July, it might mean that she saw inside Harry that part of Voldemort's soul (who was born on New Year's Eve)
Not a fan of the books
Daniel Radcliffe said in one interview that he has not read a single book of the entire saga completely. In fact, when he tried to read Philosopher's Stone, he got so bored that he didn't finish it.
The end of the story
There was only one person who knew the complete Harry Potter story line, besides JK Rowling. It was Alan Rickman. Sometimes he acted different as instructed by the directors because "he knew something that nobody else did"
Real life story
There's only one character entirely based on a real person, JK Rowling, said, and it's Gilderoy Lockhart's. She never named the real life person, of course, but some people think it's Jorge Arantes, her ex-husband
JK Rowling created the Dementors based on her experience, when she suffered a severe depression, and the cure to a dementor attack is chocolate, of course, because that always cheers her up
JK Rowling said during an interview that she based Hermione off of herself, and that's why Hermione's Patronus is an otter, her favorite animal. That's also why sometimes this character is labeled as a Mary Sue
JK Rowling had to change her name because the authorities from the editorial thought that kids were less likely to read a book wrote by a female. Of course, that was nonsense
The K from JK Rowling's name doesn't stand for nothing, actually. Some people think it is based and from the beginning letter of Rowling's grandmother's name, Kathleen. That would be so cute.
Rowling used some characteristics of her own life when she was creating these magical characters. For example, she and Harry share the same birthday, on July 31st, and both Dumbledore and her like to eat sherbet lemons
Rowling invented the House names while being sick and breathing inside a paper bag during a flight. She loves inventing names, and writes unusual ones in a notebook she keeps all the time
Hagrid and Harry
Rowling said during an interview that she had within the first year of writing the book the idea of the final chapter, or at least the part where Hagrid carries Harry out of the forest, during the Hogwarts' battle
Jack Russell Terrier
Ron's Patronus is a Jack Russell Terrier, a dog, and we know that Harry's Patronus is a stag. In the Prisoner of Azkaban, it's mentioned that James Potter and Sirius Black were animagi, and they could turn into a dog and stag.
The scene in Deathly Hallows where the gang entered Bellatrix Lestrange's vault at Gringotts so they can steal the Horrocrux required the creation of 210.000 coins! What would you buy with all that money?
Daniel Radcliffe used a huge amount of props while filming the entire series. He used 160 pairs of glasses, and broke 80 wands because he was always playing with them
Richard Harris, the actor who portrayed Albus Dumbledore the first two movies of the saga, only accepted the role because his 11 year old granddaughter threatened to never speak to him again if he didn't
The props used in the entire saga have a variety of importance and price. For example, most of the books used in Dumbledore's office are actually phone books, but the Gryffindor's sword was a real one, purchased at an auction
By the time the Deathly Hallows first move was launched, Daniel Radcliffe made a play called Equus, where he appears naked. The poster of the play appears in the café scene in London
The four founders
When Lord Voldemort was creating the horrific Horrocruxes, he used three items from the founders of Hogwarts: Ravenclaw's diadem, Hufflepuff's cup and Slytherin's locket. He didn't succeed at finding one from Gryffindor
JK Rowling said during an interview that she almost killed Ron and Arthur Weasley instead of Tonks and Lupin. In fact, some chapters were leaked only a few years ago, and she got so mad she almost felt the desire to kill Harry and Hermione, too
Speaking of difficult names and pronunciation, the "t" in Lord Voldemort's name is silent. Also, Rowling had Hermione spell her name, since many people didn't know how to pronounce it
Proffesor McGonagall was a great Quidditch player, until her final year when she had an accident and was forced to quit playing. Her name appears listed in one of the Quidditch trophies in the Philosopher's stone movies
A Little bit of everyone
The three main characters represent one of the other houses as well, besides Gryffindor. Harry was torn between Gryffindor and Slytherin, as we know. Hermione's intelligence make her a truly Ravenclaw, and Ron's loyalty would make him a great Hufflepuff
JK Rowling foreshadowed the idea of a basilisk being hidden within the Chamber of Secrets when the entire school Harry was the heir and the Weasley twins teased him with the idea of having a "fanged servant"
Pranksters for life
Want some more foreshadowing? Both Sirius and Fred Weasley, famous pranksters of their own generation, used the Marauder's maps, died while laughing, victims of the Dead Eaters. Oh, right in the feels.
JK Rowling considered some of these alternative names for the Goblet of Fire: Harry Potter and the Fire Goblet, Harry Potter and the Death Eaters, Harry Potter and the Three Champions
Harry Potter and Tom Marvolo Riddle are family related, it appears. Since they are some sort of distant cousins! Both of them descended from the Peverells. You can read all about it on Wikipedia
A truly Gryffindor
Harry always had many doubts in whether he should be a Gryffindor or a Slytherin. But the truth is quite clear from the very beginning. Harry's most used spell during a fight was the Expelliarmus spell, that comes out with a red light. Red represents Gryffindor. Crystal clear.
Minerva's name comes from the Roman goddess of wisdom. Remus was one of the founders of Rome, who was fed by a wolf. Sirius is a constellation associated with a dog.
It's a well-known fact that Proffessor Trelawney is the descendant of Cassandra Trelawney. This character might be related to the great Greek seer, Cassandra, who was cursed by the god Apollo so no one would believe her predictions.
Seven is a truly and really important number in mythology and science. It is established in the wizarding world that seven is the age where most experts think that magic will reveal itself in a wizard or a witch.