For fantasy to really work for an adult audience, one of the things its needs is the suspension of this disbelief. This does not mean that the audience believes what it is seeing is true, but that the audience has been persuaded to stop actively disbelieving it is not true.
For me as an adult fan of fantasy films, LOTR attained this very well. However the Narnia films died on their feet, because I just could not believe in the film for the duration of the story. I could not look past certain aspects and put my disbelief to one side for a moment.
For myself, the suspension of disbelief requires two things:
Firstly, it needs to be a world is consistent and in which the fantasy setting has an ethos and a mythology and does not stray too far from that central mythology. When Santa turned up in Narnia, what little suspension of disbelief I had was instantly shattered. However in comparison, Peter Jackson’s Lord of the rings knew its mythology, stuck it to its mythology, and I was therefore able to immerse myself within the story.
Secondly, the world has to be believable and consistent with its own laws of logic. Jackson’s take on middle earth was by no means realistic, but everything worked (more or less) with its own mythology. Characters had roles and limits and worked consistently within them. When I accepted the premises which the world worked in, I was asked to believe nothing which was unreasonable.
Narnia on the other hand was a complete mess. It had no tangible internal logic and was impossible to believe. A major factor in this was the capabilities shown by characters who were small children. It took a fantasy and made it into a fairy tale for me.
It is much easier to maintain a suspension of disbelief in the written form where images are much more controlled by our imagination and detached from the world we know than it is on the screen where imagery is created for us and reminds of our daily reality by its very visual nature.
This is my concern for Game of Thrones. The setting is dark and violent and does not take prisoners, but as the books continue, some of the major characters are nothing more than young children , who in any earthly situation would be out of their depth. It is easier for us to maintain that suspension of disbelief in the written form, but will it survive the translation onto the big screen where we see in essence teenagers taking to the fore?
The answer comes not from me but from the man who coined the phrase, Samuel Taylor Coleridge. He said what is needed to maintain that suspension of disbelief is “ … a human interest and a semblance of truth…”. If the human side of the story can be kept alive and well, and the essential nature of the characters believable (especially the children), we should be able to see this potential problem fade into background”.