Writing really is the study of human beings and how they express themselves. Every opportunity you can - EXAMINE (like a being from another world) how the humans that surround you express the vast array of emotions that color our daily lives.
You’re right, some people are withheld -
the merest lift of an eyebrow says volumes about what's going on in
their heads. Other are like fireworks displays - about everything;
love, hate, happiness, boredom it all gets colored with some
incredible display of emotion. The films that affect us the most are
the films where the writer has created a VISUAL language that relays
even the tiniest detail of a characters INNER landscape (emotional
landscape) to us, the audience.
So, give some thought to the multitude of NON VERBAL bits of
communication that are peppered throughout your day... your child
opens it's arms wide for a hug when you come home; but doesn't SAY
anything. Your boss merely crooks a finger in your direction and
disappears into her office. You know you've been summoned. At a chai
shop, a handsome fellow merely meets your eyes over the lip of his
cup, which mostly hides his smile of acknowledgment.... you TELL ME
what that means!!
Start cataloging in your mind the various ways people SHOW
us every day what they're thinking... instead of using words. Do this for a
month and you will be on your way to understanding one of the key
elements of screenwriting. Show - don't TELL what the characters are
I hope this helps. And I do wish you luck with your screenplay . Let me know when you’ve completed your script. I’d love to read your work!