I should note also that I am not going to talk about every difference between the books and the movie. I expect there to be differences—books can't be adapted verbatim, for reasons of time and storytelling. For instance, in the movie John Carter is given the name Dotar Sojat meaning "my right hand" by Tars Tarkas. In the books, Dotar and Sojat are the names of the first two warriors Carter kills and thus his Barsoomian name. There is not a lot of value explicating this; the writers used a name from the source, which often is enough to get a giddy sigh from the fans of the original work, and they are able to suitably adapt the name to add more meaning to the story they are trying to tell. In this case, we don't have to have pages of Carter and Tars Tarkas fighting together to see that they build up trust and friendship between them, the movie spares the viewer time by having Tars Tarkas announce his dependence on his new comrade, by naming him "my right hand."
One detail in the books that make even their fans grimace is the method by which John Carter travels from Earth to Mars. Frankly said, the reader has no idea how it happens and neither does John Carter:
As I stood thus meditating, I turned my gaze from the landscape to the heavens where the myriad stars formed a gorgeous and fitting canopy for the wonders of the earthly scene. My attention was quickly riveted by a large red star close to the distant horizon. As I gazed upon it I felt a spell of overpowering fascination—it was Mars, the god of war, and for me, the fighting man, it had always held the power of irresistible enchantment. As I gazed at it on that far-gone night it seemed to call across the unthinkable void, to lure me to it, to draw me as the lodestone attracts a particle of iron.
My longing was beyond the power of opposition; I closed my eyes, stretched out my arms toward the god of my vocation and felt myself drawn with the suddenness of though through the trackless immensity of space. There was an instant of extreme cold and utter darkness. (A Princess of Mars)…and John Carter opens his eyes on the planet Mars. But using this method in the movie would bring hoots of derision and monumental eye-rolling from critics—Carter got to Mars by wishing himself there? Instead, the screenwriters take other elements of the story and expand them to fill their need: the priest class of Barsoom has the ability to travel between the planets and Carter stumbles across one of their way stations in the Arizona desert and is accidentally transported to Mars. This method is more palatable to a mainstream audience and also opens the door to further plot development, allowing this priestly class, the Therns, to become the overarching villains of the story and provide a threat that could support a franchise of movies. Also, by couching the plot change in terms familiar to the audience familiar with the books, the screenwriters placate that same audience somewhat. Thus the story change maximizes the benefit to novice viewers and minimizes the pain for fans of the books, deriving the most advantage overall.
Of course, it could be argued that the movie could have remained true to Edgar Rice Burroughs's original vision, but that would have pushed the movie closer to an independent sort of movie, one suitable for art houses. Filmed that way, John Carter could have become a cult favorite on the art house circuit though the odds on that happening are pretty long and the profits pretty small, especially when compared to a potential movie franchise. Just by choosing to go the blockbuster route, the crew narrowed down the parameters of the kind of story they could tell.
Another fairly big change which worked pretty well for the movie was in the character of Dejah Thoris. Bearing in mind that the original novel, A Princess of Mars, was published in 1912 and was generally intended for an audience of boys and young men, Dejah Thoris as written is really not more than a plot point. She's beautiful and in need of rescuing, giving John Carter a reason for his subsequent actions. Burroughs makes it clear that she is smart and well-trained in swordplay, making her something more than just an ornament, a model of the strong woman that Carter needs by his side, but she isn't developed very far as a character in that first book. In the movie, however, Dejah Thoris is not only smart but something of an academic (Carter refers to her regularly as "professor") and still also a swordswoman, capable of not only defending herself but engaging in witty banter as she does so. Modern audiences would have been disturbed if Dejah Thoris was as flatly portrayed as she is written, and given that a potential franchise needs support across all demographics, a solid female role model can only help, especially in the young girl and teen girl segments. And frankly, it's pretty hard to read Dejah Thoris without wincing at the stereotypical portrayal of her from the time period. This updated version seems a much better match for John Carter, especially to audiences with experience with relationships, which the intended audience for the books surely never had. I freely admit to being totally messed up into my 20s about how men and women relate in part because my first impression of it came from the Burroughs books.
But on the other hand, the attempt to make John Carter himself more accessible to mainstream audiences doesn't work so well. In the books, Carter is a stoic warrior, a man of few words and little humor, but generally radiating charisma. He is the greatest swordsman of two worlds, unbeaten and unbeatable. But that sort of uber-competence doesn't work with movie-going audiences; for some reason, we want to see our heroes with flaws. The plot goes out of its way to demonstrate his ability as a fighter and his ability to strategize in its time in the Arizona desert, but then, somehow, Carter seems to forget most of it when he gets to Barsoom. The movie's John Carter is somewhat impatient and selfish, traits that belong nowhere near the character as he is written. The books' John Carter fights on Barsoom because he sees injustice and because he can make a difference, and while the movie's John Carter ultimately arrives at the same determination, it takes a lot of work to get him there. And while it may make his character more accessible, it also makes him weaker and distinctly less heroic.
One difference that was somewhat mixed in its success was the portrayal of Woola. The novels make it clear that Woola, a beast called a calot and the Barsoomian analog of a dog, is perhaps John Carter's most steadfast companion on Barsoom, the archetype of the doting watchdog. When John Carter shows appreciation, even affection, when Woola originally leaps to his defense, Woola commits himself to Carter's protection, never wanting to leave his side. The movie does a decent job of showing this, but then takes it perhaps a bit too far by making Woola a comic bit. First, Woola is ugly, described in Burroughs's writing as nothing so much as frog the size of a Shetland pony but with ten legs. The movie plays up the ugliness by letting us see often into Woola's hideous mouth, wherein is hidden his huge blue tongue. Being a faithful dog, the movie pulls out the stereotype that dogs apparently are incapable of life without drooling and slobbering over their owners, and it is made that much funnier by the dog using its huge blue tongue to show its affection for John Carter. And while calots are fast animals in the books, in the movie, Woola runs at ridiculous speed, leaving trails of dust behind it as though it were more Barsoomian roadrunner than dog. I understand why this relatively minor change was made—Woola offers some levity and appeal to a younger audience, and cynically, if it succeeds, then perhaps every child will want a stuffed Woola—but the actual carrying out of this change becomes grating to the adult audience. But the vision and conception of the beast feel dead on; the movie just pushes what passes for personality a trifle too far.
There is one fairly large change that I have no explanation for; I can neither figure it out from a writerly point of view nor from a marketing one. The main conflict of the movie is between the two city-states of Helium and Zodanga. For some reason, the plot of the movie requires that Zodanga be a mobile city, basically a city that walks on tremendous legs all over the planet. While it has a gee whiz factor and is impressive in the idea of it, the fact that Zodanga is a walking city has no bearing on the story of the movie at all, and very little time is spent there, so that there are no long scenes of the city to wow the audience. This one just feels like change for change's sake.
So, ultimately, I generally understand why the writers changed what they did to make the movie more accessible. I can accept them though in some cases I don't really like them very much. That's the trouble with the devoted (some might say "obsessed") fan; I really wanted to see the books that Burroughs wrote in cinema. But that movie would have even worse reception than what the one we have has received. And it is the very rare moviemaker who can ignore cost and income in pursuit of a vision.