first things first, this is a well-made, beautiful, well-acted movie with a great story.
unfortunately, and perhaps predictably, it falls far short of the book. it captures some of the characters and incidents of the story very well: Rene Z. as Ruby I think being the standout.
the most biggest difference between the book and the movie though is not in character or story ( though there are those differences too ), but in sentiment. the movie is rife with a sentimentality that is just not to be found in the book.
take Ada's life as a farm worker, for example. in the book it is all about lists, got to do this then that then this then that then this then that. reading the book you got a good sense of how different our lives are now from what they were back then for most folk ( or probably in all honesty for a good percentage of people alive today ). it was a shit load of work and the devil was in the details. there was nothing romantic about it. you get a little of this in the movie, but Ada doesn't seem to change very much -- she looks as much the fashionable Charleston woman at the end of the movie as she had started out with, she doesn't look worn, tired, mangled by the day to day facts of farming. it can be a good life, but a hard one nonetheless.
so to the boys that became soldiers. the movie paints one of Teagues men as a villain, mean and cruel to his bone, enjoying the wanton killing in which he is taking part, in the end in the final standoff between inman and this man, the man says something lame like. -- I have one advantage. -- what's that, says inman. -- the confidence of youth.
--- SPOILER ---
in the book he is a boy. not unusual or particularly evil, just used to violence. as inman thinks "once again he found himself in the familiar mode of violence" ( paraphrase, it is said better in the book ). in the final standoff, it is no charicature of two gunslingers going at it, but two men caught in a bad situation both afraid that the other will kill him as soon as he gets a chance. it is about survival.
the boy shoots, inman falls mortally wounded. "the boy looked at the gun in his hand as if he had not expected it to work so well, 'they god'"
in the movie there is a long drawn out scene where the man is obviously shot dead, but it is a good 10-15 seconds before Inman starts spitting up blood still astride his horse. a typical hollywood cliche if there ever was one. two men come together a gun goes off, one of the two got shot but which one!
the other telling difference is in the epilogue. Ada in the movie, on a gorgeous day, saving a baby sheep, voice over talking about inman and how she thinks about him.
in the book, inman's name does not even come up. it's not that he is not present, but it did not need to be stated.