In order to understand my position, one must first review where Aragorn, the archetypal King-in-waiting, came from. As Ax points out, for much of the drafting of the LOTR, of course, there was no Aragorn. His role in the story was played by Trotter the Hobbit, a mysterious "ranger" whose origins Tolkien was never quite sure about (the most developed concept was that he was one Peregrine Boffin, a Hobbit befriended by Bilbo who disappeared from the Shire the day he came of age and eventually was tortured in Mordor). However, as the sequel to The Hobbit more and more became subsumed by the greater depth of Tolkien’s mythology, it became more and more apparent that this was not appropriate and that this character had a much more important role to play in the story.
Before embarking on the effort to create a sequel to
The Hobbit, the major new component of Tolkien’s mythology was the story of the Fall of Númenor. This was a critically important conception for Tolkien, and in hindsight provided the perfect bridge between the older mythology of the Elder Days that would become
The Silmarillion, and the more modern (but still ancient) story of the end of the Third Age that was
The Lord of the Rings. For anyone not familiar with the story of the development of the Númenor legends, I highly recommend reading volume 5 of the HOME series - “The Lost Road and Other Writings.” It is a fascinating story and really shows how critical the conception of the Fall of Númenor was to Tolkien.
This is obviously not the place for a synopsis of that story. Suffice it to say that it details how even the noblest of mankind (eventually conceived as the Children of Lúthien) can fall to the temptation of power unbridled. But included in this tragic tale is the idea that like the Phoenix, from the ashes will rise again a new hope, which became the Numenorian realms in exile. Yet we would later see in the story of Isildur the tragic flaw raise its ugly head again.
To make a very long story short, Tolkien eventually realized that this mysterious ranger character that he had created, but had such trouble identifying, actually was the culmination of this tale, which reached its tentacles back into the old tales and incorporated the conception of a touch of a ‘higher spirit’ elevating mankind, and that Trotter the Hobbit was actually Aragorn, son of Arathorn, descendant of the King of Men. Aragorn came to represent the redemption of mankind (the new New Hope) and ultimately, to quote Legolas, “is he not of the children of Lúthien? Never shall that line fail, though the years may lengthen beyond count.”
In the book, enough of this backstory is captured (in the text and appendices) so that Aragorn makes sense, even without the benefit of reading Tolkien’s other work (though Tolkien was absolutely right in believing that the Wars of the Jewels and the Ring should have been published together as they ultimately told one long connected story with Aragorn as in many ways the culmination of that story). However, in the context of the film, it is simply impossible to include enough of this back story to make such an archetypal portrayal make sense to anyone other than us Tolkien fanatics. This part of the story is largely told entirely through Isildur’s failure and Aragorn’s redemption of that failure, through his own journey of acceptance of his destiny.
I love the breadth and width of the tale that Tolkien weaved, and expect to find new patterns and seams in the fabric for the rest of my life. I didn't need to see the same exact story on the screen that I already have in the book(s); indeed, I believe an attempt to duplicate it would have inevitably failed. It would have been futile for PJ to try to repeat with crochet what Tolkien created with fine needlepoint. Instead, he (IMHO correctly) took the same themes and colors that Tolkien used and weaved them into a new pattern that is complementary of the original pattern. I for one am thrilled that he took this approach.
Book!Aragorn would have worked for me in the film is because I am so familiar with all of Tolkien's work, and the backstory that explains where Aragorn is at. I don't need to see his internal journey because I know his history so well, from the coming of the Edain into Beleriand, through the suffering the Edain went through in the wars against Morgoth and the union with the elder children of Iluvator, the glory and fall of Númenor, and the Numenorian realms in exile in ME, and how the Heirs of Isildur retreated into the shadows until their time would come again. Would B!A work for me in the film if I wasn't so familiar with all this? I don't think so.
Contrary to what some people seem to think, Film!Aragorn is no sissy. Other than his reluctance to accept the mantle of power that is his destiny, he is portrayed as quietly self-confident, and clearly worthy of that confidence, as he shows at Weathertop, at the Council, at Moria (both with the Watcher and in the battles with the Orcs and Cave Troll), at Parth Galen (particularly in resisting the call of the ring), at Helm's Deep, the Paths of the Dead (before it degenerates into farce), the fields of the Pellenor, challenging Sauron in the palantír, and when he resists Sauron's final temptation and charges the dark forces "for Frodo"
.
F!A is not portrayed the same way as B!A. But guess what? He doesn't need to be. Book!Aragorn lives forever as part of the work of the one and only J.R.R. Tolkien. Film!Aragorn shows another glimpse of this fascinating character from a different point of view. And this is a bad thing because of why?
Here's a quote from that ultimate purist, Christopher Tolkien:
I would be inclined to think that the original figure (the mysterious person who encounters the hobbits in the inn at Bree) was capable of development in different directions without losing important elements of his 'identify' as a recognisable character.(Return of the Shadow, p. 430.)
That's just what the filmmakers did with Aragorn, taking that "recognisable character" and developing it in different directions. In one of the promo clips that came out before ROTK was released, Philippa Boyens summed up that direction very well:
How do you assume a mantle of a King? How do you take that on yourself? How do you say I'm the one that you must follow? I think that that is what he is struggling with, because he has seen what power can do.
Tolkien had a whole backstory to explore this theme of the temptation of power. PJ and friends basically had just Aragorn. Overall, I am very pleased with what they (and Viggo - sorry vison, I think he did a fantastic job with the role) did.
My apologies to those who have seen some of these thoughts before."Spirits in the shape of hawks and eagles flew ever to and from his halls; and their eyes could see to the depths of the seas, and pierce the hidden caverns beneath the world."
).
Although I think perhaps a change in attitude isn't the same as a change in character: both L and G start off READY to change. All they need is a few months in each other's company. 
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