Movie posters are designed to promote upcoming releases in order to maximise box office takings and create a buzz amongst people to see the movie. In order for the poster to be popular and successful, it needs to attract the human eye with bright colours normally, but funny enough black and white often draw more attention.
As a graphic designer it's your job to have your creative juices flowing at all times. But realistically every now and then the well may run dry. To get rid of a creative slump, experts typically suggest listening to some inspirational music, reading a design-friendly magazine, or better yet watch movies that can stimulate the mind and bring forth creativity.