I have to admit, I've done this same exact thing. I'll read a book, get excited for the movie, and then be disappointed because the actors weren't the ones I pictured or didn't have the same exact mannerisms as the character in my head. Every person walks into that movie theater with a different actor in mind, a different setting, a different idea of what the movie should and shouldn't be. What we have to realize is that with all of these differing perspectives, only two will be shown: the director's and the screenwriter's. We can't all be the screen writer or director, and they can't include every single minute detail. All they can do is represent their preferences for actors, settings, and what should and shouldn't be included.