Whoever is the photographer for movie posters doesn't get enough credit. The portraits are perfectly effective at communicating the movie character's personality. Often, you can infer what the movie's storyline is just from the movie star's portrait on the poster.
Consider the following examples:
What do you know about the character simply based on the poster? You can feel their personality through the image. When you see the portrait, you probably can list several adjectives to describe the character. That's impressive!
As photographers, we should follow the movie-poster-creators' lead. With the portraits we create, we should be able to feel the subject's personality through the image.
How do it do it? First, I think of the adjectives that describe my subject. Is she bubbly or serious? Shy or confident? Athletic or bookish. Quirky or proper? Most of us can read someone after a short conversation.
I keep those adjectives in my mind as I'm directing my subject in how to position her body. The adjectives affect how I pose her feet, how I turn her hips, how she turns her shoulders, how she tips her head, where she looks and what kind of expression do I try to illicit? The adjectives drive the entire session. Again, I want someone to feel my subject's personality through the image. I want to communicate who she is.
I'm sure that this is how movie posters are created. I'm guessing a producer meets with the photographer and describes the character - only then does the photographer create the images.
I encourage you to think of the adjectives that describe your subject throughout the session and direct him/her with those words in mind. Be sure that, anyone looking at the portrait, will think of those same words. Your photography will be better if you do.