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Horror movies and thrillers need to be your homework, because editing is finally the place where you get to create the real suspense. You could spend years studying editing, but a quick master-class of your favorite movies will do. For each scene, pull out a notepad and write down:  The length of cuts. Do they use long, slow shots, short and quick cuts, or a combination of both? When do they use each? How long is the scene, and at what point does each key event happen. You'll be surprised how similar this is in many movies. What are the music and sound cues? When do they come in, rise, and fall away? What is the lighting like? How does color, tone, and brightness affect the mood of the scene? Pay attention to the length of the cuts, and how the juxtapose against each other. While this rule isn't hard and fast, longer cuts tend to build dread and suspense, with shorter cuts breeding action, excitement, and confusion. This is a good way to play with tension and release -- a key to good suspense.  Watch how some movies "fake" the release, making you expect a scare, but actually providing nothing terrible (like a cat jumping out). This puts your audience on edge without blowing the big scare. Remember -- suspense comes from worrying about what is going to happen next, or seeing what is happening and being unable to stop it. Note how your favorite movies build this horror. Check the climactic basement scene near the end of The Silence of the Lambs for a master course in long takes punctuated by quick bursts of action. Sound is actually one of the best, if not the best, way to create tension and suspense. Why? Because it hints at things the viewer can't see, increasing worry, confusion, and dread. From floorboard creaks to the whistling of the wind, chirping crickets to sudden, heart-stopping silence, you can mine a lot more tension out of subtle sounds than most movie viewers ever realize.  You can find thousands of free sound effects online with a quick search. For serious movies, consider paying for access to a professional sound bank. Silence is key to suspense, especially when broken by a nerve-clenching noise like a sharpening blade, slowly opening door, or sudden breath or whisper. You can find hundreds of copyright-free suspense songs on the internet, generally using violins, piano, and ambient noises to build quiet dread. When using music, try to be sparing -- you want your scares to come from visual and sound design, using the music as a final touch to pull it together. Keep it in the background, barely perceptible, to subtly impact your audience without pulling their attention from the scene.  No Country for Old Men famously used no music at all, which created its own unique, silent suspense.

Summary:
Watch as many suspenseful scenes as you can get your hands on. Use the length of each shot to build, and then release, tension. Spend as much time on sound design as visual cuts. Cue up the suspenseful music, letting it build slowly throughout the scene.