A great book has a powerful and consistent voice, a voice so unique and essential that it can draw you into the story and help you remember what's happened so far even if you've put the book down for a week. To create such a voice, define the vocabulary and syntax of the narrator.
But that's only part of it. Any person can say anything or feel anything. Its always about how they say it. There's a certain cadence or rhythm. Elusive, abstract and moody.
It's like salsa. Quick quick slow. Quick quick slow.
What is your narrator's rhythm?
Silly silly sad goofy. Silly silly sad goofy.
Stuck-up stuck-up embarrassed stuck-up humble. Stuck-up stuck-up embarrassed stuck-up humble.
These patterns can happen in a couple sentences or take a paragraph each.
Its all about reaction. Let's say your character is feeling sunny. How long does it take him to be cynical? A voice that maintains an optimistic and bright tone for only half a sentence creates a very different character or narrator than the voice that stays ecstatic for an entire scene but then crashes hard.
Anyone can say anything (within their vocabulary and syntax range of course.) Anyone can feel anything. How long do they let themselves feel it? What is the pattern of their mind?