There's a psychological term I haven't seen cited by any of the myriad pundits queueing to offer advice to would-be writers. I find this strange, because it is fundamental to a character's personality. Establish this simple measure, and all other traits cascade from it in logical progression. It's all you need to know.
The term 'Locus of Control' was first identified in 1954 by the American psychologist Julian B. Rotter. Today, it's widely applied in clinical, educational, industrial and organisational psychology. So, why not writing?
The concept is breathtakingly simple. Does your character believe they control their world, or do they believe their world controls them? Obviously, it's rare, bordering on impossible, that a character is all internal or external 'locus of control' - there's a spectrum. Regardless of where they are on the scale, people are predominantly one or the other, internal or external. But, of course, we exaggerate our characters for dramatic effect.
When writing, consider whether characters are controlling or controlled; more precisely, do they feel they are controlling or controlled?
Here's an extreme example: a sociopath controls the lives of his victims. You'd assume he was way over on the 'internal' range of the spectrum, but he feels controlled by inner voices, making him an extreme example of 'external.' His destructively controlling psychopathy is an overcompensation for his overwhelming feeling of lack of control.
So, does the character you are shaping on the page believe they have the potential to control the world (internal) and shape their own destiny, or do they feel they're the hapless victims of uncontrollable events (external)? That simple choice will inform everything from their habits, obsessions, relationships, mannerisms, body language, facial expression and, ultimately, actions.
Is their mentality that of predator or prey?