There is something to be said for leeway as far as novel-to-movie adaptations go. However, left unsaid, and often heavily implied, is the unfortunately lesser observed notion of keeping with the spirit and tone of the source material; in this manner, The Desolation of Smaug fails more severely than does Sean Bean in living until the end of a movie.
I can tell you that, certainly, I am a fan of Tolkien's work. I adore the books of the original trilogy, as well as the corresponding movies. When I read The Hobbit, I liked it very much as well – but if you were to show me the two current movies with which the novel shares its name, I would likely question whether the director had any actual contact with the novel. Throughout The Desolation of Smaug, only very few scenes and characters appear roughly based upon the novel – there are some barrels, there is a hike through Mirkwood with its corresponding spiders, and, unlike the previous installment, there is actually a fucking dragon in this movie. Unfortunately, and rather noticeably, in straying from the tone of the source material to such an extent, The Desolation of Smaug never quite makes up its mind about what kind of movie it wants to be. Is it trying to be funny, as a comedy of sorts? It it fantasy-adventure like the original trilogy? Or maybe, is it an action film? Well, the answer is irrelevant, because the movie utterly fails in all three departments: the humor entices no laughter, the action lacks tension, and the fantasy comes off as uninspired.
So yes – as far as the comedy goes, the humor is indeed derived directly from the script; but not to the writers' knowledge, the audience is often inclined to laugh at the movie instead of with it. I lost track of how many times a joke based on something found in a bathroom was made in the first Hobbit, and although the humor in this movie draws from markedly fresher sources, the wit involved never surpasses the kindergarten level; and while it's true that The Hobbit was written by Tolkien for children, it would be exceptionally insulting to so settle and refuse the notion that children are capable of understanding more than the over-abundant slapstick that dominates this movie. If anything, it is apparent that Tolkien thought the opposite was true – as his childhood was during the time Beatrix Potter was being actively published, it can be seen throughout the novel of The Hobbit that eloquent language was commonplace. In terms of the movie, and through Peter Jackson's influence, the dialogue throughout is devoid of the smooth-spoken conversations apparent all throughout the novel. Instead, and perhaps as the pinnacle of all insults throughout the movie, the audience is continuously subjected to clichéd dialogue more reflective of stereotypical fantasy tropes – nowhere in the novel did I ever recall thinking of having read or heard such generic dialogue. In doing this, Jackson destroys the Tolkien-esque idiosyncrasies present in the dialogue of the novel, and correspondingly so, causes nearly the entirety of the conversations in The Desolation of Smaug to remind the audience that they are watching a movie.
The action fares no better. Any character with whom the audience is familiar from the original trilogy will be obviously known to not die; in fact, it is the case that none of these characters can even be injured. Much in the same way that the third Star Wars prequel was obvious in the way it was going to end, The Desolation of Smaug supplies the audience with not a single mystery. However, unlike the aforementioned Star Wars episode, The Desolation of Smaug cannot provide interesting action. Whereas the first Hobbit installment laughed away at the rules of physics, often inviting characters to survive falls that would kill a normal being a hundred times over, this second movie does the metaphorical equivalent of reversing gravity – and often, it is literally the case as well, as a more prominent action scene features various elves accomplishing such stupendous physical feats that eventually I came to wonder if I was really watching a shooting gallery. As a highlight, it should be pointed out specifically that Orlando Bloom manages to balance himself on the heads of two dwarfs in separate barrels inside a raging river, simultaneously firing an endless volley of arrows into moving targets in front, to the side, and behind him, while not necessarily ever needing to see said targets in order to hit them, and so hitting them in the head, at every occurrence, without fail. And this theme of elven ninjas is consistent throughout the movie, having seemingly supplanted Gandalf as the premiere deus ex machina – after the elves are introduced (unwittingly, as a deus ex machina), nearly every other transition into "a scene with elves" starts with Legolas or his girlfriend appearing out of thin air to kill some orcs. Over the top is an understatement; the action itself merits the supposed "fantasy" label this movie is sure to receive, because there is no other excuse as to how the characters throughout this movie do what they do, the way they do it.
The fantasy element of the movie is perhaps its weakest component. Those who have read the novel will not recognize the characters or settings they came to like while reading, because all Jackson offers is CGI on top of more CGI. Even despite the setting of this movie, where every other spectacle necessitates CGI by virtue of its fictitious nature, I found myself underwhelmed, realizing that the given vista I was admiring was probably the work of some nerd in a dark room. Or maybe it was really a bona fide effects technician – the point is that eventually, CGI fails to impress simply because it isn't real. Conversely, the original trilogy prominently featured the beauty and majesty of New Zealand, and quickly immersed the audience with long, overhead shots and steady camera panning; and as far as The Desolation of Smaug goes, the modernized fast-cut fast-zoom technique is all that's used in filming the action and environment. Ultimately, this causes the action scenes to feel like a video game, but more importantly, it causes the random glimpses of the environment to not provide a sense of scale or direction – what good is seeing the top of a snowy mountain if I have no reference point for its height or what lies near it? Certainly I can recall the fire-beacon sequence of the original trilogy, where the eye was skillfully drawn around the screen, prompted to look all over the mountain sides for the next light – whereas in The Hobbit, the audience is given (or rather, subjected to) a cursory peek and then is immediately cut away to some tirade.
So supposing Peter Jackson (or whoever is relevant here) was to sell the rights to film J.R.R. Tolkien's The Hobbit, and supposing further that the names of characters and settings and events were changed, a lawsuit over copyright infringement would have no legal basis.