This isn't just a movie, it's an interactive movie in which you choose your own adventure. Some scenes throughout the movie will give you a choice, in some places, the choice you pick will give you a number. At the end of the movie you add all these numbers up and that will determine your rank.
Not all choices throughout the game will offer you a rank but they offer you another way to do something. You cannot see every scene through on one viewing of the movie, in fact if you don't play at least 4 times through you will miss at least 40 scenes. In the big mission some choices will cause you to die and you will be taken back to the choice again, not back to the start.