Film posters are an important way to attract audiences to watch a film. They must clearly show the genre and plot through images and colors that represent the narrative. Effective posters for the films "When a Stranger Calls" and "Unknown" use images that indicate the films are thrillers - a scared woman on the phone and a character not knowing their identity. Additional text elements like the film title and quotes or ratings from critics are also included on posters to provide more information to potential viewers.