This recommendation comes from my favorite film in the 2013 48 Hour Film Project in Denver. Surprise was made by my friends at Through the Fire Productions. The line that had to be used for the film that year was, "why don't you do it?" and while most teams find any convenient place to throw the line in, The film makers used it to create an extra creepy moment. Check it out:
Everything in the short film was expertly done down to the costumes, location, makeup, and acting (Tiffany won best actress for her performance). The cinematography, editing, and music creates a paranoid atmosphere that puts you into the head of the main character, making the climax all the more unsettling.