Most of the movies created nowadays are movies derived from a book story. Harry Potter editions, Twilight saga, and the newly produced Hunger Games series are just a few examples of this trend. For every movie that has come out after the whole world’s scrutiny, a film review is made.
The common mistake among writers with regard to reviewing a film derived from a book is that they tend to make comparisons with how the book was written and how the movie justified the book. This is no good at all.
Just like selling a house ads, film reviews are written to educate viewers and promote the movies. A movie may have been derived from a book but that does not mean that the assessment of the movie should be dependent on how a reader was able to imagine it when one is still reading the book. The movie and the book, though containing the same content, are still two different things.
You have to consider that not everything that is in the book should be shown in the movie. It is the director’s judgment as to what to include or not for the sake of the film. There can be a sort of argument here that would make critics say that it is always good to abide by the book since the plot is from it. Again, considerations have to be made.
When writing a film review, be fair enough to make judgments not based on the result of the comparison. Judge the movie by how it was produced, quality wise. Encourage readers that for every movie that has gone out of the shelf, there is the benefit of the doubt to be served.
You can only be an effective writer if you are able to sort your judgment out. While it may be good to have the book as the reference, give the movie a chance.