How do I create a character that's reclusive but interesting?
Writing really is the study of human beings and how they express themselves. Every opportunity you can, really observe (like you’re 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 emotionally withheld; the merest lift of an eyebrow says volumes about what’s going on in their heads. Other people are like firework displays about everything – love, hate, happiness, boredom all gets colored with an 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 character’s inner landscape or emotional landscape to us, the audience.
So, give some thought to the multitude of non-verbal bits of communication that are on display throughout your day. Your child opens his or her arms wide for a hug when you come home, but doesn’t say anything, just the gesture. 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 acknowledgement to you. You tell me what that means!!
Start cataloging in your mind the various ways people show us every day what they’re thinking – without using words. Do this for a month and you will be on your way to understanding one of the key elements of screenwriting. Show us, don’t tell us what the characters are thinking.
I hope this helps. And I do wish you good luck with your writing.