I used to write a lot of film reviews. I found that the hardest part was getting the necessary information from the credits. It can drive you crazy trying to write down names and title while the credits are rolling--especially in the dark! In some cases I would be sent a "press kit" which contained everything I needed to know about the actors and the names of the characters they played, the director, the cinematographer, and on and on. But in other cases I didn't have this information, and I found that Weekly Variety always furnished complete details and reviewed every movie that came out in the U.S.
If you have that information in print, then you can just sit back and enjoy the movie. If you have access to a good library you ought to be able to refer to the pertinent issue of Weekly Variety in their periodicals room. The excellent reviews in that publication--which is like the bible of show biz--will give you models to follow. The librarian at the reference desk or the one in the periodicals room ought to be able to show you how to find out what issue of Weekly Variety contains the review of the movie you are reviewing. In some cases, you might have to use microfilm, but that is something worth learning to do if you don't know already. They will show you how to work the machine. That is one of the things they do all the time. Usually you can take a photocopy of the review right off the microfilm machine. The New York Times also offers pretty detailed information, especially on big productions like the Harry Potter movies. These too would usually be on microfilm.
There is nothing unethical about taking factual information from other reviews. Most of it comes from press kits sent out by the publicity departments of studios anyway. For instance, some reviewers will tell you what other films an actor has appeared in, what other films the director has directed--or there might be some unusual information that would make your review more iinteresting, e.g., there was a big rainstorm and some scenes had to be shot in the rain, or whatever. You might also learn about how much the film made during the first week or two.
No doubt you can also obtain a lot of factual information about films via the Internet.
Look for the MacGuffin. Who wants what? Why can't he or she get it? Why do we care? Did you like the film? Why? Why not?
This is a writing assignment that I also make my middle school students write, and this is how I tell them to set it up:
In your first paragraph, introduce the topic that you are reviewing, which in this case is a Harry Potter film. I recommend choosing your favorite one to review, or the one you have seen most recently and are more familiar with. Include some specific information about the movie and characters, like a brief summary of the set-up of the movie, without giving away any of the key action or 'spoilers.' Be sure to include what kind of audience this movie is geared toward. Who would enjoy it? Who would not enjoy it?
In this next paragraph, you want to establish your criteria for evaluating the movie. Criteria are reasons why you think the movie is good or bad. Some criteria for the Harry Potter movies could be: special effects, characters, acting, music, setting, story line. Then use those criteria to make a clear statement of judgment on the movie, like: "I definitely recommend Harry Potter and the Deathly Hallows because of its believable characters, amazing special effects, and powerful storyline."
Then using your criteria of why you liked the movie, give examples and back it up with evidence from the movie. For example, if you say you really like the special effects, make a body paragraph where you specificaly talk about how amazing the special effects were. The more specific you can be, the better. Describe what happens, but again, be careful not to give away the plot!
In the final paragraph, discuss your emotions and feelings from watching the film. Be honest about your experience. Was the movie as good as you hoped it would be?
Finally, restate your original recommendation of whether you recommend the movie or not, along with your criteria of why it was or was not successful.
*The biggest mistake movie reviewers can make is to give TOO much summary and not enough criteria. Do not retell the entire plot, but instead discuss the reasons why you genuinely enjoyed the movie.
Before you start writing your own film review, I recommend you read some popular ones on the web to get a feel for what you want to write. Determine what sort of tone you want to establish: snarky? serious? sarcastic? After that, figure out which aspects of the movie you want to focus on: character development, use of special effects, how close it stayed to the original novel, etc.