How do you adapt a book set half inside its characters' heads?

And can you not somehow adapt a book written later than 1900 that isn't about a detective?

The pair struggled to find a way to adapt the book to fit the conventional three-act film script structure.

I know it's not normally like that when you adapt a book.

Much work had to be done to adapt the book, for example there are only around four lines of dialogue that ended up in the movie.