It was a very long road following the Harry Potter series, stretching across 14 years. The following will give you an idea of what books were out compared to which movie was out. This is handy for folks interested in the history of the phonomenon. For instance, when the first film was released, the fans going to see it could have only read through Goblet of Fire.
Hermione's surname was originally Puckle. She was planned to have a younger sister, until Rowling realized it was a bit too late to start mentioning her.
Furthermore, the Potters originally lived on an island close to where the Puckles did. In this version of the story, Hermione's father was the one who pulled Harry out of the wreckage of his parents' house.
At King's Cross, there is a false archway labeled "Platform Nine and Three Quarters" kept well away from the actual platforms — complete with the rear end of a luggage trolley protruding from the brickwork.
In fact, the description that Rowling gives of the barrier between the platforms doesn't sound anything like what currently exists at King's Cross, for the very good reason that she was thinking of nearby Euston station when she wrote it, and only discovered her mistake later. It's not even correct at Euston; at both stations, there are train tracks between platforms nine and ten. In any case trains travel to Scotland from both stations, though by different routes.
Rowling considered giving Dudley a Muggle-born wizard child for the epilogue, but in the end decided that no wizarding traits would ever survive contact with Uncle Vernon's DNA. Shame, that would have been a lovely little twist.
Gilderoy Lockhart is based on an actual former acquaintance of JKR's. She doesn't believe he'll ever realize it's him.
Ginny and Harry's roles in Chamber of Secrets are reversed in Half-Blood Prince. First there's the Unrequited Love Switcheroo and then there's the fact that in the respective installments both are seduced by an evil book which they dispose of (or at least try to) after association with it causes them to endanger one or more people.
There are a lot of other parallels with books that are mirrors of each other, given book 4 as the centre — 3 and 5 both focus a lot on the Marauders and Dementors, 2 and 6 focus on Voldemort's past, and 1 and 7 have a lot of focus on the night Harry's parents died.
Dean Thomas was originally going to be Ron's cousin, and would've fulfilled much of the role that eventually went to Neville Longbottom. Also, Word of God revealed that Dean is a half-blood, whose father was killed for refusing to join the Death Eaters. His Muggle stepfather adopted him, explaining why Dean believes himself to be Muggle-born.
Pansy Parkinson was based on girls who used to bully JKR in school.
The first name of Salazar Slytherin comes from the Portuguese dictator, António de Oliveira Salazar.
The core of the Elder Wand is the tail hair of a thestral.
Arthur Weasley was actually originally intended to die of his injuries in Order of the Phoenix, but Rowling had a last second change of heart after realizing that Arthur was the only positive paternal character in the series. Still wanting a Sacrificial Lion however, this resulted in Lupin and Tonks getting killed off in the last book.
In a 2014 interview, J.K. Rowling admitted that she regretted making Ron and Hermione the Official Couple instead of Harry and Hermione as they were more compatible. Though she says this changes nothing and imagines Ron and Hermione would probably be alright in the end after some couples counseling
Aluminum Christmas Trees: One might think that a fumble-fingered executioner failing to sever a person's head completely was invented from whole cloth by JKR. As it happens, Jack Ketch did it twice.
There used to be a rumor about a gratuitous sex scene in the last book. Everyone heard that rumor (from everyone else), and all the sources claimed to have heard it straight from the author. Although Ginny's "birthday present" arguably came close, there was no onscreen sex. A video on The Onion about Rowling including a date rape in an upcoming Potter book may have had something to do with the rumor.
It was rumored that Rowling had confirmed Harry Potter and the Green Flame Torch / Pillar of Storge / Fortress of Shadows as the title of books six/seven. She responded by suggesting Harry Potter and The Toenail of Icklibogg.
J.K: I am trying very hard not to feel offended that anyone thought this was possible. 'Storgé', for crying out loud. Come on, people, get a grip. 
There was an article Harry/Hermione shippers would sometimes cite which claimed Rowling had said Harry would develop "more of an interest in pal Hermione" in book 5. Nobody was able to trace this to an actual interview, and it ended up being Jossed.
Rowling never said that she may have subconsciously let the name "Harry Potter" from the Troll series seep into her story, regardless of what Phelous or any number of other people may tell you. What she actually said is that she took the name "Potter" from a family she was friends with as a child, and "Harry" simply because she has always loved that name.
There's a quote floating around on the Internet which confirms the fanon belief that Nagini is the snake Harry set free from the zoo, specifically "Yes, it’s rather funny, really, that next to no one realized the snake that Harry set free in Philosopher's Stone turned out to be Voldemort's final Horcrux, Nagini." It never happened. There's also a scientific hole in the theory, incidentally. Harry set a boa constrictor loose, which in the movies, was a python. Neither one is venomous or has fangs. Nagini is and does.
There are persistent rumors that Rowling said "In a fight between a wizard and a muggle with a shotgun, the muggle will win every time." This quote has never been traced, and whether muggles or wizards would win is a continuing debate.
Rowling said in a 2014 interview with Emma Watson that Ron and Hermione would have needed marriage counseling and might not have been perfectly compatible. Many media outlets reported that she said pairing Ron and Hermione was a mistake (a fair enough interpretation) or that she said Harry and Hermione should have ended up together (which she didn't say at all).
Jossed: Blaise Zabini, first named in the first book and a popular OC Stand In, gets his first proper introduction in the sixth. Fans who had written Blaise as white or female were outraged that he turned out to be a black male.
A lot of fanfics ended up being victimized when it turned out Ginny is not a shorthand for Virginia Weasley, but Ginevra. Which could have been seen coming, considering that Ginevra is a British name (and an old one at that, as is popular among wizards) while Virginia is mostly an American one besides people like Virginia Woolf.
Name's the Same: Rowling liked the last name Evans, so she gave Harry's mother that maiden name. Later, when writing Order of the Phoenix, she named an unimportant character who Dudley beat up "Mark Evans". The fans went nuts trying to find the connection between the two... but Word of God says she's sorry, it was just a name she made up on the fly and should have known better.
Obvious Crossover Method: [Crossover character] raises Harry instead of the Dursleys might be the single most common crossover mechanism for Potter Fanfic.
Golden Snitch: An aspect in a game or contest that, if met, will put the game out of reach for their opponents, named after the game-ender in Quidditch.
Hold Your Hippogriffs: An Unusual Euphemism that references a fantastical element in the story, named after the half-griffin/half-horse animals in the series.note Note that they existed before appearing in Prisoner of Azkaban.
Hufflepuff House: A team or group that is basically unimportant within the main plot, named after the fourth house at Hogwarts.
Invisibility Cloak: An item, usually clothing, that renders the wearer invisible, named after Harry's own invisibility cloak.
Mad Eye: When a character has one eye larger than the other due to losing their mind, even if momentarily. Named for Mad-Eye Moody, who had one eye notably larger than the other and was rather eccentric.
Muggles: Ordinary people who are unaware of the magic/supernatural goings-on of the plot, taking the name of "non-magic folk".
Muggle Best Friend: An ordinary person drawn into the extraordinary world by their connection to the main character.
Muggle Born of Mages: A character born to people with powers who did not inherit them. In the series, this is called a Squib, and is not quite a "muggle".
Muggles Do It Better: A case where someone demonstrates that the mundane, often Real Life solution to a problem is much more effective than the magical one the main characters try to use.
Muggle Foster Parents: When a super-powered character is raised by normal parents. Harry himself is an example.
Pensieve Flashback: A flashback in which the character actually experiences the scene from the past, walking around in a three-dimensional space but unable to affect the scene before them. Named after Dumbledore's depository for his 150 years of memories, which can be entered in order to experience memories in this manner.
Secret Keeper: A character who keeps the supernatural nature of another character a secret, named after the result of the Fidelius Charm, which hides the caster and requires they choose someone as a Secret Keeper, and only they can reveal their location.
Tom the Dark Lord: A phenomally powered villain with a mundane name, named after the man who would change his name to Voldemort.
Wronski Feint: Feinting at an obstacle and pulling up at the last minute. This is a classic Quidditch move.
J. K. Rowling originally wanted to include a character named Mafalda in Goblet of Fire. She would be a distant cousin of the Weasley family, sorted into Slytherin, and she would have served as a rival to Hermione due to her high intelligence.
Rowling was asked permission to make a Harry Potter musical by Michael Jackson! Sadly, she turned him down. Rowling has always fought to keep Harry Potter as British as possible.
In early 2014 stated that she regretted the Ron and Hermione relationship. She implies that this revelation came while writing the book but decided to follow through on what she was already building up with Ron and Hermione. In the end she imagines that there relationship would still work after some couples counseling.
Word of God: Quite prevalent as Rowling gives a lot of extra information through interviews. Also, the FAQ she keeps on her personal website.
Worthless Yellow Rocks: Supposedly Nigel Reynolds, who was the arts correspondent on the Daily Telegraph, met Rowling and she handed him a first edition of Philosopher's Stone. He took it to his office, skimmed it, and threw it away. Copies of the first edition, of which only 500 were printed, are now worth thousands of dollars.
Harry Melling, the actor who played Dudley in the films, is the grandson of Patrick Troughton.
Conversely, the actor who plays Dean Thomas, Alfred Enoch, is the son of William Russel, the man who played Ian in Doctor Who.
If you were to watch the theatrical cuts of all eight Harry Potter films back-to-back, it would take nineteen hours and thirty-eight minutes. The extended cuts would add twenty minutes to that, which is with released extended cuts existing for only the first two movies.
One is American (Columbus), one is Mexican (Cuarón), and two are British (Newell and Yates).
The order they directed the films in is the same order their names would be listed in if organized in alphabetical order by last name.
Newell (born 1942) is the oldest person to direct a Harry Potter movie, being sixty-three when he directed Goblet in 2005. Although Yates has the most recent birthdate of all the directors (1963), the record for directing a Potter film at the youngest age is a tie between Columbus (born 1958 and directed Stone in 2001, making him forty-three) and Cuarón (born 1961 and directed Azkaban in 2004, also making him forty-three), whereas Yates was forty-four when he directed Phoenix in 2007. J. K. Rowling is younger than all the directors, being born in 1965.
Columbus and Yates are the only people to direct more than one Potter film, with Columbus directing two of them and Yates directing four of them. As such, Yates has directed the entire second half of the series.
Azkaban is the only movie in which the title doesn't appear over a cloud background (the title did appear over the customary cloud background in the trailers, but not in the actual movie). Out of the ones with cloud backgrounds, Chamber is the only one in which it's not a stormy cloud backgrounds. Stone and Prince are the only ones which include lightning in the cloud background. Chamber and Phoenix are the only films in which no scene takes place between the appearance of the Warner Bros. logo and the title.
The Deathly Hallows films don't really divide the book in half. More accurately, Part 1 covers about two thirds of the book and Part 2 about one third.
After Deathly Hallows came out, it was revealed that Alan Rickman (who plays Professor Snape) had had a conversation with J. K. Rowling about his character while filming Sorcerer's/Philosopher's Stone, and was the only other person besides Rowling who knew that Snape had been in love with Harry's mother. He based his entire performance off this conversation. (He was also JKR's personal favorite to play Snape.)
Deathly Hallows: Part 2 is the most financially successful Potter film, being the only one to pass the billion dollar mark. The least financially successful was Prisoner of Azkaban, which is "only" the thirty-second highest grossing film of all time. Hallows: Part 2 is also the most critically successful entry in the series (96% rating on Rotten Tomatoes) while Order of the Phoenix was the least critically successful (78% rating).
The two halves of Deathly Hallows were written and filmed as one giant four-hour film. It was worked out in editing where to put the break between Part 1 and Part 2. Originally, the break was tentatively planned to occur after the Trio is captured by the Snatchers, but it got moved to after Voldemort recovers the Elder Wand. This means the battle at Malfoy Manor was originally intended as the beginning of Part 2, but instead became the end of Part 1.
Warwick Davis has played two characters within one film twice in the series — in the very first movie and in the very last movie. In both cases, he played Flitwick and a Gringotts goblin.
First two films: The credits are plain white words on a black background. The starting music is "Harry's Wondrous World", which is not heard in subsequent films, either in the credits or elsewhere. The second film is the only movie in the series to have a scene after the credits.
Third film: First film in the series to use Creative Closing Credits, which in this case is an animation of the Marauder's Map. The music is a Credits Medley, titled "Mischief Managed!" on the movie soundtrack.
Fourth film: The credits appear on pieces of paper, resembling the scraps of paper thrown into the Goblet of Fire. Another Credits Medley, which opens with a Rearrange the Song version of "Hedwig's Theme". The credits track is not on the official soundtrack, which remains the case for the rest of the series.
Fifth film: The words appear on a parchment-like background in the same typeface as Umbridge's proclamations. The music is another Credits Medley, opening with part of "Dumbledore's Army".
Sixth film: The words form out of ink-like clouds the way images in the Pensieve do. The music opens with "Fireworks", which was composed for the previous film. Last film to use Creative Closing Credits.
Seventh film: A plain black background is used for the first time since the second film, but with the words being golden and metallic-looking. The music is "Obliviate".
Eighth film: Same as the seventh film. The opening music is an unaltered version of "Hedwig's Theme".
Voldemort's headquarters during the Battle of Hogwarts got changed from the Shrieking Shack to the boat house because Stuart Craig wanted to get to show the interior of the boat house at least once in the series.
The first line of the first movie and the last line of the last movie were both spoken by someone named "Albus":
Albus Dumbledore: I should have known that you would be here... Professor McGonagall.
Albus Potter: [after Harry asks "Ready?"] Ready.
A common joke among Italian Harry Potter fans is that Snape is the secret identity of the singer Renato Zero. Why? Well, here's Renato Zero◊.
There is a rather famous series of tapestries decorating the Gryffindor common room, looking rather much like this.
Before appearing in the Potter films, Harry was the young David Copperfield in a 1999 BBC television drama. It was his first screen work before the Potter phenomenon. In fact, this is where Chris Columbus and David Heyman first spotted him and thought about casting him as the boy wizard; also, Maggie Smith a.k.a. Professor McGonagall, who had also starred in David Copperfield with him, would recommend him to Columbus that he play the role of Harry.
People who saw Twilight were also probably more likely to have gone "Hey, it's Cedric Diggory as the vampire!" but obviously Robert Pattinson's newfound fame from Twilight might have eclipsed his potterfame.
Watch the film adaptations and then watch Shakespeare in Love. The stuttering man that delivers the opening and closing words of the play at the end is same actor that plays Mr. Weasley, Mark Williams. Likewise, go watch a few episodes of the Eleventh Doctor's tenure and you'll see him as the father to Rory Williams, who interestingly bears a lot of similar personal traits to Ron.
And the cheerful comic relief nurse? Professor Umbridge.
Common wisdom has it that if a British actor did not themselves appear in the Potter films, they have shared a screen with someone who did — probably several times. This may, in fact, be absolutely true.
Coldmirror did some wildly popular German dubs of the 1st ("Harry Potter und ein Stein"/"Harry Potter and a Stone"), 2nd ("Harry Potter und der geheime Pornokeller"/"Harry Potter and the Porn Cellar of Secrets") and 4th ("Harry Potter und der Plastikpokal"/"Harry Potter and the Goblet of Plastic") Movie.
There was some uproar about Clémence Poésy (Fleur Delacour) appearing topless in a French movie, even though it was before she was even cast in the film.
Emma Watson has let it be known that she's willing to do nudity in other films, which means it's just a matter of time before the Moral Guardians scream about Hermione Granger showing her dirty pillows in some movie or another. Watson has also berated by Moral Guardians for the shocking crimes of... going to a nightclub and drinking. There's also a Paris Hiltonesque crotch shot, which those guardians must been specifically looking for in order to find it.
Jamie Waylett (Vincent Crabbe) got this in 2009 after being arrested for possession of marijuana, even though Crabbe isn't a particularly "wholesome" character to start with. Worse still, Crabbe was written out of the last two films because of this. Insanity, especially because Crabbe dies in the book. In the films, Goyle dies instead of Crabbe.
Harry Melling, who plays Dudley, has lost a huge amount of weight over the years (to the point of having to wear a fat suit for the final movie) and is barely recognizable from his younger self. He's publicly stated he is happy about this so he can avoid this issue in his adult career as an actor.
Fake Brit: Almost averted, by fiat of Rowling herself. The cast is from the UK, with a few Irish actors such as the original Dumbledore, Richard Harris, Fiona Shaw as Petunia Dursley, and Evanna Lynch as Luna Lovegood (who is presumed British, though Lynch uses her natural accent).
A few slight examples: In the first film, the goblin Griphook was played by American Verne Troyer, but his voice was dubbed over by Warwick Davis. And Chris Columbus cast his daughter Eleanor in the non-speaking role of Susan Bones.
Fake Irish: In a strange example, Rhys Ifans (who is Welsh) played Xenophilius Lovegood with an noticable Irish accent to fit in better with his on-screen daughter Luna Lovegood (who is played by Irish actress Evanna Lynch).
And even stranger, and rarely acknowledged, example is Michael Gambon, who admitted right off the bat to adopting a slight Irish accent in honour of his predecessor — it's only obvious at certain times, such as when he loses his cool with Harry in Goblet of Fire, but it does give the impression that Dumbledore was born and raised in Ireland, or at least spent enough time there when he was young to pick up the accent and retain traces of it for a full century.
Rupert Grint, as he had an obsession over being Ron once he realized how many similarities he had to the character.
I Am Not Spock: Zigzagged with the adult actors. Some generally fit this trope well (Fiona Shaw, David Bradley), others seem to be on the fence (Robbie Coltrane, Jason Isaacs), and others completely avert this (Gary Oldman, Ralph Fiennes). Then you have guys like David Tennant who have been Spocked into a completely different role. On the other hand, all of the child actors with the exception of Robert Pattinson are mostly identified with their characters.
Limited Special Collector's Ultimate Edition: Apart from two-disc editions, there's the Ultimate Editions, with an extended cut and many goodies in the packaging. The Limited Wizard Edition features almost all of the special edition content including Ultimate Edition content and more in a $500 30 disk set including everything on both blu-ray and DVD, It also comes in a large elaborate case.
Most famously, Richard Harris and Michael Gambon as Dumbledore.
Throwaway characters tend to get recast after they get important, such as Parvati Patil, Angelina Johnson, Lavender Brown, Katie Bell, and Pansy Parkinson.
The character of Voldemort supplies two examples. The present-day Voldemort was first played by Richard Bremmer/Ian Hart and then by Ralph Fiennes. Sixteen-year-old Tom Riddle was played by Christian Coulson in the second film and by Frank Dillane in the sixth.
And unless that was an uncredited Ciaran Hinds having undergone mounds of plastic surgery in film five, it looks like this is happening to Aberforth in the final films.
The Fat Lady was played by Elizabeth Spriggs (film 1), and then by Dawn French (film 3).
Lavender's recasting especially sticks out, as she actually changed race for the sixth film — she suddenly becomes Caucasian when she was previously black. In the books, her race was ambiguous.
Tom the barman, from a normal looking man, to... uh, bald Quazimodo?
In the seventh film, Hermione's parents are played by different actors than the ones who played them during their equally brief appearance in the second film. (One of them beingCatelyn Stark!)
Role Ending Misdemeanor: Because of Jamie Waylett's little run-in with the law, evidently Goyle rather than Crabbe gets the honors of an on-screen death in the last film.
The line "my children/younger relatives begged me to take the part" and/or "my children/younger relatives would never forgive me if I refused the role" shows up with distressing regularity in cast interviews. Yep, Emma Thompson too.
The initial concept for Lucius Malfoy was of a man with short blond hair and a pinstriped suit. Jason Isaacs argued that Malfoy, being a wizard supremacist, would want to look as different from a Muggle as possible. He put on a blond wig and velvet cloak and suggested a cane for grand gestures, leading to the now-iconic Lucius Malfoy look.
In Chamber of Secrets, when Harry and Ron are disguised as Draco's minions, Harry forgets to take off his glasses. Draco points them out and Harry says he was just doing some reading. Draco's response ("I didn't know you could read") was an ad-lib by Tom Felton.
In Chamber of Secrets, the bit with Harry going "Don't worry, I will be" was Daniel Radcliffe deliberately channeling another Harry.
The reason Lucius Malfoy tried to outright murder Harry after he freed Dobby in Chamber of Secrets was because Jason Isaacs ad-libbed the first curse that popped into mind, and it happened to be the Killing Curse.
Sean Connery was offered the role of Dumbledore, but turned it down because he's not a fan of the fantasy genrenote This is mentioned in an installment of Saturday Night Live's Celebrity Jeopardy! with the SNL version of Connery claiming he turned down Harry Potter just for the chance to harass Trebek again..
Patrick McGoohan was also considered, before Richard Harris was cast. Oddly enough Mc Goohan died after principal filming of Half Blood Prince had ended, so they may have only needed a new Dumbledore for a few bits of Deathly Hallows and Kings Cross.
After Richard Harris's death, Richard Attenborough, Christopher Lee, Ian McKellen, and Peter O'Toolenote Who actually received the blessings of Harris' family were each considered for the part of Dumbledore.
The idea of splitting Goblet of Fire into two films was given very brief but very serious consideration.
Order of the Phoenix
Helen McCrory was originally cast as Bellatrix Lestrange, but had to drop out shortly before shooting began after discovering she was pregnant, and was replaced by Helena Bonham-Carter. She was later cast as Narcissa Malfoy in Half-Blood Prince. Ironically, shortly before shooting began on Half-Blood Prince, Carter also learned that she was pregnantnote You'll notice that Bellatrix is wearing looser robes in HBP, as opposed to the form-fitting ones of OOTP..
Anna Friel, David Thewlis' long-time girlfriend, lobbied hard for the part of Nymphadora Tonks.
Naomi Watts was rumored to have been cast as Narcissa Malfoy, but was later revealed that she had never even been considered.
John Williams was supposed to come back to compose the score, but scheduling conflicts prevented that.
The role of Helena Ravenclaw was originally going to be offered to Kate Winslet, but her agent shot it down before it even got to her.
A musicalWhat Could Have Been: The movie series could have very well featured an example of Crowning Music of Awesome by The Boss himself, Bruce Springsteen, reportedly called "I'll Stand By You." Springsteen wrote this ballad sometime between 1998 and 2000. He was inspired when he was reading the novels to his youngest son Sam. Sometime in early 2001 Bruce made the song available to director Chris Columbus, who at the time was shooting the first of the Potter movies. A Springsteen recording of the song was filed with the US Copyright Office on 13 Jun 2001. However, the Springsteen song was ultimately rejected due to Harry Potter novelist/creator JK Rowling's contractual stipulation that no commercial songs of any type be used in the Harry Potter film series.
During the casting calls for the first film, Tom Felton auditioned for the role of Harry first, and then Ron, before being offered the role of Draco.
Chris Columbus was hoping to come back to direct the rest of the films after the third film was released. Although that did not happen, he has said that he is still proud of the work done with all of the films.
Word of Saint Paul: The last film suggests a romance (or at least a pair of reciprocal one-sided crushes) between Neville Longbottom and Luna Lovegood, which is a widely fanon supported couple, but not one that happens in canon. Matthew Lewis, who plays Neville, asserted the two had a brief fling, but ended up marrying their canonical partners.