The danger with theme is not that it will get changed unnecessarily, but that it will simply get lost along the way. The idea was, those who can fight, have a responsibility to step forward and fight. And the fact that people do this, despite their fear, is a tribute to the human race. This entire concept was simply lost in the film adaptation, and so the film came to be about, essentially, nothing. Again, here's another reason to work with existing material.
Characters are often the key element in any adaptation.
You're going to see the characters. If you can get the characters right you've won half the battle of doing an adaptation. For each film, much of the King-written speech patterns were retained for the dialog, thereby retaining the 'sound' of the novels. This was enhanced by the choice to use voice-over narration, which benefited both movies. It's a great device, if it can be used, in that it effectively retains the voice of the original author.