Purpose of a film poster

I have researched the purposes of film posters to ensure I design a poster which is effective. From this research I now know I need to ensure my poster creates a hype and I could consider doing this by including the names of the actors and the star rating as the audience will think it is worth watching. Also,  I need to include the BBFC rating of our film on the poster to target our actual audience which is men and women ages 15-25 years old. This is so people are aware whether or not the film it is suitable for them to watch. Furthermore, I now know I need to ensure I make the genre clear with the use of images and text, however, images and text along with colour and the film institution will also represent the films target audience.