To come up with such good insights, one needs to study a movie really closely. According to AMC Filmsite, watching a film for more than just pure entertainment sake is usually enhanced by viewing it with an informed awareness of how it works. Only if you strive to be a 'critic' (in the best and most general sense of the word), will you improve your understanding of the craft.
When analyzing a movie, you should start by gathering information about it. When was it released? Who are the main stars? Who directed it? Which studio produced it? Preferably, you should ask yourself these and other similar questions before hitting the play button.
AMC Filmsite advises watching a deserving movie more than once. Another thing to consider is doing it with others so that the experience may be enhanced with discussions afterward.
During the first viewing, you should become familiar with the basic plot, main characters, important action, and so on. In other words, you should acquire a brief synopsis of its story including the time and place, the characters and their interactions, and the movie's main themes without taking notes.
The second time around, try paying more detailed attention to what you're seeing. Take notes and make observations, begin to think in a more concentrated way about the movie's structure, acting, directing, the camera shots and angles, and other technicalities. Prepare yourself to listen carefully to the production's soundtrack and how the music, songs, and score enhance the general mood.
Most importantly, strive to maintain objectivity to what is being presented. Don't prejudge it from the things you've heard about it, your preconceptions about who worked on it, and so on.
A few of these and you'll be writing 'terrible' movie taglines in no time!
