It just wouldn't be a thriller/horror without a massive knife. This prop is used to anchor the film as such a genre, and it is also incorporated to make it obvious where the blood in the film is from and how it has been shed. Although, the knife is introduced after quite a while, so the audience wonders where the blood has come from. In addition, it also shot in black in white when portrayed, reminiscent of the classic Alfred Hitchcock moments. The black and white also represents the dream and flashback in which the knife is shown.