The 1st & the most important for designers to make a movie poster, is attention

This doesn’t have to be achieved with provocative pictures or flashy graphics, although given their advantage at grabbing attention, it’s no wonder Hollywood’s/Bollywood's /Tollywood's  turned to them en masse.

By using the film’s characters or a major plot point, designers can establish some level of plot while still gaining the attention of anyone that views the poster.