When I am looking at a poster for a movie I would expect to see the main characters that are throughout the film. Those characters being dressed in the same attire that they wear throughout the film as well as props that may go along with those particular costumes or characters. Another thing I would expect to see is obviously the title of the movie and when it was coming to theaters. The background of the characters should match the title or what the characters are usually surrounded by in the film. The poster should be appealing to the eye and make you want to see the film.
Perhap you could do a crowded school yard with a lot of students going on with their life and laughing, bussling and then one boy who would be front and center just standing there by himself looking extremely lonely. Everyone has found and has their group of friends except him
