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The best trailers propel you along, leaving your begging to watch more. They do this through a perfect attention to rhythm. Trailers are often edited like music videos, meaning that the cuts line right up with the beat of the song. Once you know what song you're using, return to the video and cut it so that each transition fits your backing music. Can you tell a short story in the trailer? Think of great trailer's you've seen and how they function. The start with a bang, slow down and explain the conflict, then gain steam until a climactic final shot. This is, in essence, the structure of many real movies. Having this mini roller-coaster ride can make your trailer more compelling, which makes the movie seem more interesting as a result. Not all trailers need to show clips from the actual movie, though most do need a few seconds of footage. The trailer for Eternal Sunshine for the Spotless Mind features a fake commercial for the memory-wiping technology. The Social Network Trailer starts with a minute or so of someone browsing Facebook. Sometimes, starting with clips or images that aren't from the movie helps build suspense, capturing audiences used to hearing "In a world where..." every time. Too many trailers try and fit in everything-- the best jokes, scenes, and plot points-- and end up ruining the entire movie. You want the audience to be curious about the movie, not like they just sat through the spark notes version of it. The best trailers will focus on 1-2 elements, exploring them well, while still hinting that there are other aspects of the movie worth seeing. This is mostly about feel, but you should always ask yourself while editing: "Does this give too much away?"
Focus on the rhythm, not just images to keep the audience engaged. Make the trailer a mini-movie. Use original footage instead of clips from the movie. Retain an aura of mystery.