I found this amazing article about the
'anatomy of a movie poster'. The best information was written by the
author. I can only add you should consider to start working on your
movie poster in pre-production. Six months in advance or if you wish even more.
You do not have your Actors nor your locations? A large amount of
'Hollywood' movies start by making 'concept' posters for promotional
purposes. And after the production time is done (main photography),
they develop new posters in post production. And they usually end up
selling both posters to the public (and generate even more revenue).
Wanna shoot your next movie? Consider starting with its first poster...