The first response is a good response, but I would like to say a bit more because a film, while it tells a story, does so in a different medium from a book.
There are many elements that make up a film, and you can create standards for each and assess each element according to that standard.
Movie elements include acting, scenery, music, lighting, photography, and editing. In each case, you will want to examine whether the element contributes to or detracts from the music. For example, if a movie that takes place in the year 1400 has a soundtrack of rap music, would the music make the movie better or worse? In a movie in which closeups would be important to see people's emotions, if everyone is filmed from far away through filters, what does that do for the movie?
Certainly, storyline and character development are important elements in a movie, but a movie is a more complex production than a book in many ways, and I would hope that you could look at some of the other elements, too.
Good luck to you!
When you have a big task, make it small. Don't talk about the whole movie at once; see how it is made up of parts that unfold chronologically.
You may not know this, but almost full-length movies have three acts. And since most movies are about two or so hours long, 120 minutes, each act is about forty minutes. (Actually, if you've ever read a screenplay, each page is about a minute, and so each act is about forty pages long).
So take your movie, and watch it to see how the acts unfold, how they led one into the other. Discuss each act for what it does, how it futhers the plot and how it reveals things about the characters and the choices they make. Watch especially the main character and note how he or she grows, changes or evolves throughout the drama. This growth or development, is called the character's "arc."
So, trace the development of the acts and talk about the arc of the main character, and you're on your way.