For many years DC comics had been searching for the right director, the right screenplay and the right concept to make Watchmen into a movie. As the 20-oughts came to a close they found their director: Zack Snyder, who had previously directed an adaptation of another graphic novel, 300. In 300, Snyder framed and filmed shots to recreate still images from the comic, so great was is zeal for faithfulness. 
When Watchmen was released in 2009, similar attention to detail and love was paid to each frame. It can rightly be called a faithful adaptation. Yet it is still different. Certain elements of Watchmen the comic book were impossible to translate to the screen and others were ignored. New ideas and images were added. With this post I will examine the differences between the two Watchmen media as far as plot, structure and backstory are concerned. Cosmetic differences will be ignored. Screenwriters and storytellers, pay attention! This post is specifically for you! With these insights, perhaps we can gain some understanding of how a graphic novel ought or ought not to be adapted to the screen.
Author's note: This post is not for people who have seen neither the movie nor read the graphic novel as it is rife with ***SPOILERS*** The insights listed here are only based on outsider's observations and may not represent the actual views of the creators. In other words, it could all be total bullshit. I guess you'll just have to take that chance. Read on.
The Tales of the Black Freighter sequences which punctuate the graphic novel are absent. In the comic, a young black kid reads Tales of the Black Freighter comics while a nearby white news vendor chatters about world events and his views thereon. Their two worlds are disjointed: the kid reads his dark pirate comic, the narration balloons and panels of which are interspersed with events in real life. He seems to pay no heed to the babbling news vendor.
The Black Freighter comic itself is a reflection of the views of many of the Watchmen's characters: that the world is a dark, unhappy joke. A sailor travels through a hellish ocean with corpses as his raft to save his family from the pirate ship that slaughtered his crew, only to beat his wife and commit murder in his resulting madness. “How had I reached this appalling position, with love, only love, as my guide?” laments the narrator.
Finally, as the kid's comic ends in tragedy, the news vendor himself laments upon how unconnected are his fellow humans and asks the kid what he's doing there. The kid says he has no place better to be. When Ozymandias' disaster descends upon New York, the news vendor and the kid clutch each other in terror, their last fearful embrace becoming the connection they lacked in life. The mood, if not the message of the Black Freighter infects Watchmen, but was neglected for the film. The reasons why are obvious. The sequences, as important as they are, do not advance the main plot, nor do they serve as backstory. Most adaptation requires cutting and editing in pre-and-post production, and it makes sense that The Black Freighter had to go. The comic reader and the news vendor are there at the end, getting blown-up, but that is as much homage to the Black Freighter as could be spared. Zack Snyder must have found this omission painful because he lent his talents to the production of The Black Freighter as a separate entity from his Watchmen movie. A link can be found to the animated short here: http://www.youtube.com/watch?v=kdVDztzynjU
Next, Watchmen contains several sections which were originally included at the end of issues which are newspaper clippings, Rorschach's psych profile, and quotes from “Under the Hood”, a book by the original Nite Owl, amongst others. These sections serve as valuable backstory in the comic book, exposing the events and intentions of the costumed heroes of days past. 
Film is a visual medium, but it ain't that kinda visual. Showing too many words onscreen would be death by boredom and showing all that backstory would make for a nine-hour movie. Instead, the Watchmen movie attempts to convey to the audience as much meaning and backstory as possible through imagery, particularly in the opening credits. It is a poor substitute for the sheer amount of information in the graphic novel, but it is the best that film can do without boring the audience. 
Also serving little plot in the graphic novel are the sequences that take place in the New Frontiersman's newsroom. Once again, while serving as flavour for the universe of Watchmen, little happens there. The only event of note to the main plot is the discovery of Rorschach's journal in the crank file at the end. This event, and this New Frontiersman event only, is shown in the film. What was changed in the film? And what was new material? Watchmen the movie was a labour of love for its crew. They could not resist adding their own touches to the final canvas.
The Watchmen film includes many more aspects of life in this alternate 80's than the comic. For one thing, there is far more President Richard Nixon. Where Nixon has two scenes in the comic, he and his distracting false nose have at least four in the movie. The film also contains more 80's nostalgia. At the beginning, the Comedian watches the McLaughlin Group on television, for instance. Interestingly, 80's nostalgia wins over the comic's backstory in some cases, such as the automobiles. In the comic, Dr. Manhattan has allowed for the mass production of electric cars and airships by his ability to create lithium in vast amounts. In the movie, while airships are seen flying about, the automobiles appear to be standard gas-powered models available in the 80s.
One of the most noticeable additions is the amount of violence. It has been over twenty years since the publication of Watchmen. At the time, comic books were criticized for the amount of violence therein. However, since then, the action-loving public has developed a bloodlust that is difficult to slake. The bloody 90's have made their mark on film. As a result, Watchmen the film is filled with gore and fighting, not that the comic lacks, but just more of it.
Examples? The Comedian's death is protracted and involves the flinging of knives and a smashed hand. Instead of being escorted off government property, the Silk Spectre II instead attacks a government agent, slams his head into a sink and escapes. The assassination attempt on Adrian Veidt in the comic results in the shooting in the chest of Veidt's secretary. In the film, while she doesn't die, she does get shot through the leg and loses two fingers, while an old plutocrat is shot in the chest and head. When Roschach is framed for the murder of Moloch and jumps out a second-story window to escape, instead of landing painfully on some garbage cans and being quickly subdued by police, he rolls and keeps fighting, landing eleven ninja-like blows before he is pacified. Another way in which the movie is “amped-up” is that time is truncated. While the motif of a ticking clock is powerful in the graphic novel, the clock is ticking faster in the movie. The novel contains a scene where Richard Nixon discusses the escalating nuclear threat with his advisors. The scene ends with Nixon saying, “I think we'll give it a week, gentlemen, before bringing out our big guns. After that, humanity is in the hands of a higher authority than mine. Let's just hope he's on our side.” In the film, Nixon delivers the same speech, but says, “Dr. Manhattan has two days. After that, humanity is in the hands of a higher authority than mine. Let's just hope he's on our side.” In theory, the film's creators hoped to add more tension to the story by making time even shorter.
I leave the biggest change for last: the nature of Ozymandias' plot to prevent nuclear catastrophe. The novel's story has him kidnapping intellectuals and genetically engineering a monstrosity. This monstrosity is huge and has the cloned brain of a powerful psychic. Using teleportation technology imperfectly crafted from Dr. Manhattan, Ozymandias transports the creature into the middle of New York, where the teleportation process kills it. In its death throes, it lashes out psychically, killing millions. Amongst the images it telegraphs into people's brains are hints of an alternate dimension. Believing that the world is under inter-dimensional attack, the forces of earth unite to stop a common foe. 
Ozymandias' plot differs in the film. There, he uses Dr. Manhattan's power to create what the world believes is a limitless energy source. Instead, he uses its power to launch terrorist attacks in metropoli around the world. The attacks bear Dr. Manhattan's energy signature, and the world, believing he is responsible, unites against him. What could possibly be the reason for changing the nature of Ozymandias's plot? I have two theories, both of which could be true. The first is that the film's plot is simpler. The comic's plot requires a lot of explanation and, frankly, it would confuse a lot of moviegoers. Film once again earns its reputation for dumbing-down the plot.
But, secondly, the film's plot is slightly more compelling than the comic's. When Dr. Manhattan realizes that Ozymandias has killed millions to save billions, he must not only accept the greater good, but he also must overlook the fact that he is being demonized by a deluded world. The fact that he doesn't care about being the world's boogeyman is very revealing of his character. This revelation about his character, while it is based on events which don't happen in the original story, is true to the novel.
O ye screenwriters and storytellers who have stayed with me to the end of this post, what have we learned about adaptation? If you know anything about writing Hollywood movies, probably nothing you didn't already learn elsewhere. When you go to adapt a novel, graphic or otherwise, for the big screen, keep these guidelines in mind:
1. Eliminate plot threads that don't influence your main characters.
2. Don't bore your audience with too much exposition.
3. If you are writing something historical from the living past, play on nostalgia.
4. Add more violence.
5. Add a ticking clock. If there already is a ticking clock, set it forward.
6. Make it simpler.
7. You can always make your characters more involved in the plot.
Does that sound formulaic? Maybe cynical? Maybe like it's capable of destroying great works of literature by turning them into screen-trash? There's some merit in those criticisms.
I believe in good writing, not necessarily Hollywood writing. The Watchmen graphic novel is sublime. It's film adaptation is one of the more faithful that I've seen, and it's very entertaining. But it didn't move me. I am truly at a loss to describe why. I don't think it's because it became more Hollywood-ized, either. As often happens with adaptations, something was lost in the medium transfer.
However, if you do your job as a writer well, changing the medium of a story from page to screen can make a masterpiece. It was watching The Fellowship of the Ring that inspired me to become a writer. The Lord of the Rings, Pride and Prejudice (the 1995 version), Master and Commander and Rosemary's Baby are among my favourite screen productions. Some productions, like the Sharpe series, are actually better on screen than the page. When I read anything I am always adapting it for screen in my mind and considering what scenes will work and which won't. Adaptation can be very rewarding for audiences and lead them to reading the original source material. That's a good thing. If some dude decides to read Watchmen because he saw some lady's fingers getting blown off, I'd say the movie has achieved a noble goal. http://pharoahphobia.blogspot.com/
