The book and movie complement each other. The book explains the more confusing parts of the movie including the starchild and the final "Jupiter and Beyond the Infinite" sequence which the movie conveyed through spectacular imagery. The reason for this was that the book was written as the same time as the film.
A lot of the protagonist Leonard's unexplained, highly material history in Memento is found on the Memento website.
Star Wars took this a bit too far in the Prequels. Nothing in the films really tells us what the Sith are, aside from the fact that they use the Dark Side of the Force. So Revenge of the Sith loses something in that we never really learn what the revenge is for, without knowing the details from the Expanded Universe. But considering that's kind of an important part for the motivation for every other thing Palpatine does, that's not something to just assume the audience doesn't need to know.
"And don't any of you shit heads tell me that it was explained more in the novelization or some Star Wars book. What matters is the movie. I ain't never read one of those Star Wars books, or any books in general for that matter. And I ain't about to start. Don't talk about them stupid video games or novels, comic books, or any of that ***ing crap. I've seen enough of that shit."
Revenge of the Sith begins with Coruscant under attack and recently "kidnapped" Chancellor Palpatine a prisoner on General Grievous's ship. While it's not strictly necessary in order to understand that setup, the first series of Clone Wars cartoons (the Genndy Tartakovsky ones, not the CGI ones) actually showed his kidnapping. The last episode of that series ends the moment the movie begins. The same cartoon has the introduction for General Grievous; going only by the movies he appears out of nowhere and his presence is never explained.
On the flipside, if the viewers read up on the Visual Dictionary by the time The Phantom Menace is released, it would also essentially spoil the fact that Senator Palpatine was the same guy as The Emperor in Return of the Jedi. Something similar happens with the Inside the Worlds of the Original Trilogy guide, where the portion where it shows the Emperor electrocuting Luke has the statement about "Darth Sidious' Crooked Fingers," essentially spoiling the secret of who Darth Sidious was before Revenge of the Sith unveils the revelation that Palpatine and Sidious are the same person.
The original '70's novelisation of Star Wars revealed that the Emperor's name was Palpatine in a brief foreword before the story proper even began. The expanded universe had been using the name quite casually since at least The Thrawn Trilogy.
Plus, you can just look at the actor's name in the credits. . .
The Chronicles of Riddick universe certainly applies to this. Not only are there tons of special features for the two main movies that are everything from trivia to mass info on the backstory of The Necromongers (who you would know almost nothing about just from watching), and other character's pursuit logs for the main character, there's also a 30 minute anime film that shows what happened right after the end of the first movie and introduces a major character, two video games (Escape from Butcher Bay and Assault on Dark Athena) showing how Riddick got his eyeshine and escaped from prison, leading up to the first film, and on the website, there's a point and click adventure game, an animated comic book, and collection of background info on all the characters in the first film and who they are. There's also a novelization of the first film revealing more of Riddick's past and the character's internal thoughts, a novelization of the 2nd film that has an exclusive epilogue, an exclusive mock-documentary only available on the region 2 DVD of the first film, and it just goes on and on.
Much of the background of Rose Red (the titular house featured in the miniseries) is covered in detail in The Diary of Ellen Rimbauer. Without it, its just a bunch of people going to a haunted house that hates them.
Southland Tales (by the director of Donnie Darko) apparently makes more sense when you read the comic books connected to it. Of course, since this movie extended Rule 34 to vehicles, they have a lot of explaining to do. Keep in mind that it becomes readily apparent that the film's a pseudo-sequel to Donnie Darko. It greatly expands on the character roles described in The Philosophy of Time Travel, so it's almost a manual about the manual...
Donnie Darko itself. One of the reasons it has become a cult Love It or Hate It film is that it is not self-contained at all. Nothing about the Manipulated Dead or Tangent Universe is ever explicitly (or implicitly!) stated, requiring you to read the script-book or check out the director's commentary at length to have any hope of getting the plot.
In Star Trek (2009), the motivations of bad guy Nero are only vaguely alluded to in a mind-meld flashback scene. In order to fully understand what happened, and to give the character some actual depth, you have to read the Star Trek: Countdown comic books (although the physics of Romulus being destroyed by the supernova still might have some physicists scratching their heads, even though it's further explained).
The comic also covers what the movie doesn't, how Nero's "simple mining vessel" became that humongous juggernaut of a warship.
Transformers uses this fairly prominently. A good deal of backstory for Megatron, The Fallen, Optimus, Bumblebee, and the Cybertronian civil war in general is covered in the prequel and sequel comics marketed by IDW. The absence of the information covered in these comics makes some of the decisions and motives in the films confusing, Megatron's allegiance with The Fallen being one of the most-cited among fans.
The only issue is that these comics can be nonsensical, inaccurate, or even downright contradictory to the movies. The only ones considered canon are the prequel to the first movie, which was made into a bonus DVD feature. Otherwise, they're considered a different variation of the same continuity.
The comic book prequel to Iron Man 3 explains why War Machine was absent during the events of The Avengers, showing that he was dealing with a terrorist attack in Hong Kong at the time of the invasion.
The Marvel One-Shots also serve a similar function. For instance, The Consultant shows what happened to the Abomination after The Incredible Hulk, and reveals that he was initially picked as a potential member of the Avengers before the Hulk joined up.
In Thir13en Ghosts, a lot of the 'Who are these people and why should we care?' information for the Black Zodiac isn't actually in the movie, but rather in special footage on the DVD.
Avatar has an on-line guide explaining points that were left out of the film proper.
Avatar: An Activist Survival Guide was a book released for this purpose. Some editions of the DVD and Blu-Ray include the guide as an extra.
Why they even want the 'Unobtanium' in the first place (a room temperature superconductor - restored to the actual film in the collector's edition).
It also explains how humans even got to Pandora in the first place without the Unobtanium.
When The Truman Show arrived in theaters, a companion book marketed as an in-universe supplement to the show was released which contained a copy of the screenplay as well as detailed background information about the setting and each of the various characters who appear both as "actors" and as audience members in the film.
Wild Wild West. The novelization provides some altered scenes, including introducing a subplot where West claims while surviving on his own he befriended a native shaman, who later appeared as Jim fell off the giant spider to revive him.
TRON Legacy never explains where the Iso's come from, simply stating they just "appeared." It also mentions, but never explains the Sea of Simulation. The comic book TRON: Betrayal details that the Iso's came from the Sea of Simulation. It also makes C.L.U. a much more sympathetic character.
TRON Evolution explains most of this as well, but is much more vague and less sympathetic towards Clu. For now, it appears that the comic is canon. TRON: Uprising adds a lot more detail to the time period between the coup and the second film. And even on the first film, the Novelization has a truckload of detail about the Program world that couldn't make it onto the screen. That isn't counting the discredited game and comic book that the fandom will also use as resources.
The 2005 version of King Kong has "The World Of Kong: A Natural History Of Skull Island", an art book done in the style of a nature journal, with the information in it apparently collected on expeditions that occurred after Kong was revealed to the world. It goes into great detail explaining the living habits of the various creatures (many not seen in the film itself) that lived on Skull Island, as well as explaining the island's geographical conditions (Skull Island was literally ripping itself apart).
Since The Lord of the Rings recaps the events of The Hobbit in less than a minute, a few of the plot points can be a bit confusing if you haven't read the latter novel. The reference to "The incident with the dragon" will go right over your head, you may wonder how Bilbo and Gandalf already knew each other at the start of the film, and you may find yourself thinking, "Who the hell is Gollum, and why should I care?" when he's introduced. And the ending, where an aging Bilbo leaving Middle Earth with the elves won't be nearly as emotional.
Planet of the Apes (2001) had a bizarre Gainax Ending in which Leo returns to his time, only to find that the Earth had been conquered a 2nd time by General Thade before he got there. If you visited the (now defunct) official website, it would explain that Thade accessed the Oberon's computer system while he was trapped in it and learned about the real history of the world. Afterwards, he managed to escape, fishing Leo's pod out of the swamp where it had crashed and fixing it to working order before riding off to the electromagnetic storm and arriving back on Earth before Leo did, where he staged a second rebellion.
Even worse, the VHS version of the movie claimed that everything you needed to know to understand the ending was already in the movie, showing a series of clips that apparently explained it for you. All the clips explained, though, was that the planet was actually Earth, a plot twist that had reached It Was His Sled status long before the movie came out.
The first Mad Max film has the character of the Dark One. Originally, he was Max's partner and May Swaisey's husband, but for whatever reason he was removed from the final draft, appearing only as the man they take Cundallini's severed hand to who reports it to MFP, Max's line "May, call the Dark One" when Jessie is chased through the forest, and the names "M. Rockatansky" and "The Dark One" on the Interceptor's fender (though this is best seen in promotional stills).
While it was one of the most faithful comic book adaptations, Sin City had to leave out some dialogue and a couple scenes didn't make it to the theatrical cut (though are in the director's cut). In particular, how Hartigan found Nancy's apartment, Dwight's monologue about why he can't use his own Cadillac and how the Thunderbird used to be a Cool Car, but has been abused to its present condition.
The One has a (now shutdown) website that provides tons of background information regarding the Multiverse Agency and how Yulaw came to be a villain. It reveals that he's not the first nor the last to try that. In fact, one of the "offenders" is a female assassin who is hired by a wealthy businessman in Universe Alpha (the one to actually invent interdimensional travel and the one we see twice very briefly) to track down and kill his doubles, although MVA analysts suspect she's also taking the opportunity to kill her own doubles. Yulaw has actually managed to cover up his own murders for quite awhile until another agent started suspecting him and tricked him into revealing his superhuman strength by carrying a box he stuffed with 300-pound weights (the agent paid for that by being made a paraplegic by Yulaw). Interestingly, the website doesn't provide much information on how wormhole travel works, only mentioning that it has something to do with quantum tunneling.
There are some inconsistencies with the website. For example, Funsch, according to the website, was an LA cop in Universe Beta before being recruited by MVA. The film makes it appear as if he's from a warzone where everyone is considered a combatant. Of course, it's possible that LA is really that bad in Universe Beta (although the fact that his hobbies, from the website, include fishing and restoring classical cars makes that unlikely).
The original script described Saavik as being half-Vulcan, half-Romulan. This doubles as an Aborted Arc in conjunction with Star Trek VI, as Valeris was intended to be Saavik.
The Red Shirt who dies when Engineering is attacked is Scotty's nephew, which explains why he reacts so emotionally. A scene explaining their relation was cut. The scene was used in the extended ABC broadcasts in the early 1980s, and subsequently included in the Special Director's Edition DVD.
Some people consider the reboot's revelation that Spock created the Kobayashi Maru test to be a distracting change, but subtle hints throughout the movie imply it here. Spock knows the number of times Kirk took it, and the way he finally beat it, but a captain's academy test results aren't usually shared with his junior officers. That Spock never took the test himself is also telling.
The novelization of the film reveals that Saavik and David became lovers after the events of Star Trek II, making his murder doubly tragic and his sacrifice even more meaningful (he gives his life to save the woman he loves when he realizes the Klingon is about to kill her).
For Star Trek Into Darkness, the retailer Sainsbury's has an exclusive edition with a second bonus DVD disc containing 33 minutes of extra features. In North America, the release is split into various retailer exclusives. Retailer Best Buy has an exclusive Blu-ray edition with 30 minutes of additional content, available on disc in Canada, and via streaming service Cinema Now in the United States. Target's Blu-ray edition also has 30 minutes of additional content that is different from Best Buy's. Online retailer iTunes' version comes with audio commentary for the film not available in the retailer exclusives.
The most iconic object from eXistenZ, the pistol made out of bones and teeth, is not named in the film. Background material reveals it's actually called the "gristle gun".
The tie-in prequel comic (which is presumably completely canon, since its story is credited to Goyer) reveals that Thanagar exists in this continuity — as does Kara Zor-El, whose ship lands in Canada thousands of years before the beginning of the film.
How Clark goes from a full beard to clean-shaven is never explained, though in the comics he shaves with heat vision and a mirror (or other reflective surfaces). The omission is kind of a cop-out since a Gillette ad campaign to promote the film was "How Does He Shave?"
The Graphic Novel Pacific Rim: Tales from Year Zero is a series of flashbacks about some of the characters from the movie and how they came to be part of the Jaeger program.
The company man at the end of Alien³ named Michael Bishop, the man who created the Bishop line of robots, is not named in any part of the film's dialogue, even in the Assembly Cut. The credits referred to him as "Bishop II", which only fueled the misconception brought on by shoddy editing that he's some sort of special robot with red blood.