Matt at Mere Orthodoxy discusses one point on which Peter Jackson's movie version of The Lord of the Rings seems to miss an important point of Tolkien's book. I'd like to suggest another: the cause of the Ents' going to war.
The Ents, one of Tolkien's most original creations, are characterized in the book as being, above all, not hasty. The Entmoot (a council meeting for Ents) at which the Ents decide to go to war takes a long time: at least a couple of days. As Treebeard says, "we are not hasty folk." But the meeting takes such a long time because Treebeard wants to acquaint the other Ents with the facts concerning Saruman. But, says Treebeard, "deciding what to do does not take Ents so long as going over all the facts and events that they have to make up their minds about." In the book, after deliberating for three days, the Ents decide to go to war.
This is in accord with the teaching of Aristotle; he says in the Nicomachean Ethics that "one ought to be quick to do what has been deliberated, but to deliberate slowly" (1142b).
In the movie, as many readers know, the Ents decide at the Entmoot to not go to war. The hobbits then decide, reluctantly, to return home and ask Treebeard to carry them to the edge of the forest. When they come to the edge, they discover that Saruman has destroyed much of the forest, including many trees under Treebeard's care. As a result of this discovery, Treebeard goes into a rage and summons the other Ents to war.
Now, one might claim that three-day meetings between talking, walking trees do not make for good movies. That's probably true. (In this respect, filming the Council of Elrond is much more difficult because there the viewer needs that information.) But I don't see why the movie couldn't have filmed the Entmoot as it did and then stayed true to the Ents' choice as they make it in the book.
One conclusion that may reasonably be drawn from the movie's "adaptation" is that the screenwriters think the decision to go to war cannot be the result of careful deliberation. It must be the result of a hasty action, which itself is done as a result of passion (usually anger).
Or, at least if the screenwriters don't believe this, they believe that audiences will not find the book's version convincing. But that seems to be because modern audiences do not find the method employed by the Ents of the book as a plausible way to make decisions in general. That is, the majority of people make decisions based on an overwhelming feeling. Perhaps acting decisively after careful, lengthy deliberation is too foreign to many moviegoers today.
Aristotle would not be pleased (and, in fact, would not even deign to call what the Ents do in the movie "making a choice," since he thinks that choice requires deliberation).
I don't think it's a stretch to suggest that this particular difference, though in some sense minor, reveals Tolkien's receptiveness to Christian just-war doctrine and the moviemakers' discomfort with that doctrine. Though in this judgment, perhaps I am being too hasty.