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Most of the books are indeed infilmable, you stated the reason. That's way almost all the movies are _adaptations_ of the books. Mainly the screenwriter has the infamous job to take out the infilmable scenes and hopes that the director is transposing this into art of filming. Some mood states, or emotions are thus found back into the film, but on a different artistic dimension.

There are tools to handle that, for example Ender has plenty of characters around him to help verbalize some of that internal dialogue. It's certainly hard, but it's doable with the right talent.

(Or they could do Avatar's "dialogue with the recorder" and I'll turn into a raging fanboy hate machine)

Life of Pi was unfilmable.