This series of articles is designed to help you, the fan, try and divorce yourself from your attachment to source material and judge a film on its own merits and not in comparison to another work. These tips come from my own experience. I hope they are helpful.
The aforementioned tips are all well and good when you are the fan of a standalone piece in another medium and it is being adapted into a standalone, for the time being, film. Things get more complicated when your book or comic or what have you is part of a series. Any series will have its own arc and structure in its greater tale aside from just the structure of the single volume.
This is where you might have to breathe deep and learn some relaxation techniques. If an element, say Hermione’s quest to end the enslavement of House elves, is left out of one film it will be left out of each subsequent film until it becomes absolutely, positively crucial to the structuring of the story. So some of the subplots that enrich a book will invariably fall by the wayside, which is why comparing mediums is dangerous.
I don’t want a novel that reads like a screenplay. I want detail, inner monologue, I want it to be possible to take two pages to describe five seconds of a character’s life. Each medium has its strengths and to expect a film to be a pictographic facsimile of a book is unrealistic. Sticking to the Harry Potter theme The Deathly Hallows is 759 pages long. If those pages were screenplay pages you’d be looking at a 12 and 1/2 hour movie. So even with two films telling the tale of one book you’re looking at roughly 40% of the material in the book covered.
So it’s a fact of life that the movie by necessity can and will leave things out and change things.
Conversely, you need to look at the film within the context of the series. You can compare it to past films but also bear in mind: how did it advance the story, did it up the stakes, is it leaving the table set nicely for a subsequent edition should there be one?