Googling 'movie poster' is a start but you need a more specific direction.
This is to nail down the viewers' impression and keep that direction going as you finish this.
I'm basically saying what Decroded said here but a good movie poster (the best ones) is like a good propaganda poster. It's direct, it sucks your eye to the page, and on the abstract level it's also iconic.
But it's also like a time capsule, in how it's made, what it shows, etc.
Think about stuff like, is the poster old (30s horror), or is it modern, is it a modern take on old posters.
Then take cues from that.
2- or 3-person head shots are common and generic nowadays so this looks pretty modern, and you can stick with that but you need to go further with it.
The first thing I went back and redid, was planar reconstruction of the heads.
(
Here is the wip with wireframes)
That gets me more control over and realism with the lighting, something this needs with the direction it's going.
(EDIT: Just found an even better ref for this principle than the one I did
here)
Then tweaked the sizes of the heads based on one of Decroded's edits.
Emphasized some stuff like the lighting and symbolism.
+ Fake credits.
Also there are a few kickass artists who worked on posters back in the day you can use as reference:
Drew Stuzan
Bob Peak
On the ones with big heads look at how they are arranged, it's not a triangle unless the triangle is explicit and the poster is built around it.
Also squint to see that the shapes are all different, I tried to incorporate some of that abstract thinking in the edit. (But it only really works here in the upper left.)
Hope this shows you what to do.