Film trailers, or previews, are short advertisements designed to entice viewers to see a movie. They usually run between 2 and 2.5 minutes, and the companies that produce them are very good at compressing a great deal of information into a short time. Analyzing them is similar to analyzing a longer film, except you filter it through the demands of advertising.
Examine your general emotional response to the trailer (or at least the response it's intended to evoke). It can tell you a great deal about what kind of film it is. If it leaves you smiling, it's likely intended to be a comedy. Horror films work for a sense of foreboding and dread. Summer blockbusters aim at getting the adrenal glands pumping; romances work to evoke a sense of yearning and passion. With those basic emotions as a guide, you can analyze the specific means the trailer uses to create them.
Watch for a sense of story within the trailer and the details it provides you about the plot. Though very short, trailers still deliver a basic dramatic arc: who the characters are, the obstacles they face and their development between the start of the film and the end. After seeing a trailer, you should be able to briefly describe what the film is about and the overall tone it will set.
Look at the methods the trailer uses to lure you into the theater. Trailers essentially serve as unanswered questions, prompting you to buy a ticket on opening day in order to find out how it all comes out. They can do that in many different ways: set up the storyline and then decline to discuss the finale; stress the spectacle on display through shots of the visual effects; emphasize the threat or danger the characters will face. Ask yourself what the trailer is selling you and whether that's an effective means of persuading you to buy a ticket.
Check for the presence of certain actors in a trailer. Big-time movie stars are often selling points alone, and will often be prominently featured throughout a given trailer. (Trailers sometimes make it appear as if an actor is the center of the movie, when he just has a cameo or supporting part.) Actors can further clue you in on the nature of the film itself: ensemble pieces will feature numerous different actors spread across the whole trailer; more intimate movies will center on just one or two in theirs.
Pay careful attention to the use of montage in a trailer. Montage is an editorial technique whereby multiple shots are strung together to create a unified meaning. Trailers often use them because they can achieve a given emotional effect very quickly. Watch the way the shots are assembled, the pacing of the cuts (more cuts imply a faster and more exciting film), and whether the assembly illuminates the film's subject matter or simply obscures it behind empty images.
Listen to the sound and music in a trailer. It's often used as a bellwether for the overall tone: Like montage, music can cue specific emotions very easily. Most trailers don't actually use music from the film itself. (The score is the last thing to be inserted into a film.) Whatever you're listening to likely comes from another film or a piece of classical music, so don't expect to hear it when you buy your ticket.
Check taglines and catchphrases used in a trailer. Like the other elements, the taglines are intended to give you information about the film in a brief encapsulation. If done right, they're exciting and intriguing. But the worst are more desperate or clichéd, relying on stock phrases and an enforced sense of excitement rather than genuine inspiration.
Like any other form of advertising, movie trailers can sometimes be deceptive. The tone they strike may be much different than the movie itself, and a trailer that looks spectacular may lead to a big disappointment if the film can't match the expectations it creates. Always take movie trailers with a grain of salt.