Non-diegetic sounds added in post-production, such as music, can be used to create suspense and indicate to the audience that something scary is about to happen in a horror film trailer. The document discusses using slow, peaceful music at the beginning of the trailer that gradually becomes menacing when the villain appears. During murder scenes, no non-diegetic sound will be used to build suspense, but eerie music will be played leading up to the events. Fast-paced action music will accompany outdoor scenes of the villain chasing teens, and silence will add tension during intense moments before reverting to slow, eerie music once a teen is caught.