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Differences between a film adaptation and a book it is based on

Differences between a bestseller or a clever book and its film adaptation always bring up arguments. In many cases, ordinary people or even critics have different opinions about which one is better and more likely what are the main or small differences in detail.
First of all, we need to mention that a film is already somehow prearranged because it is made after the director or the screenwriter’s imagination. While reading a book everybody is able to unleash their own imagination.
As we all know, a book, novel, drama is much longer than a two-hour-long movie. This means the filmmakers have to shorten the story just to stay in the genre and not to create it into a one-season-long episode. What is more, the screenwriter even changes the original story, miss and add different details to the film to make it even more interesting or simply just to live for their creativity.
Moreover, what really bothers me, they often change the main or supporting characters. For instance, in the original story, the main character is blonde and has blue eyes but in the film adaptation, he has dark brown hair and eyes. Obviously, it is really hard to find perfect actors every time to fit every detail into the original story but it is still very disappointing for the audience.