As a child of the '90s, I'm no stranger to epistolary stories. They were everywhere when I was growing up: Jaclyn Moriarty's Feeling Sorry for Celia, Meg Cabot's Princess Diaries, the enthrallingly haunting Griffin & Sabine; and many, many more.
Stories written in diary and letter form, and all their modern equivalents, consistently come back into style (a trend that goes back centuries), and it's not hard to see why.
These aren't the words of a distant narrator, high in the sky. These are the words of the characters themselves, right in front of us on the page, in all their intimacy.
Naturally, this still leaves plenty of room for drama and suspense. In an epistolary story, we're completely dependent on what the characters choose to share with others and how honest they're prepared to be, even for an audience of one in the pages of a journal.
There are few better ways to hear a character's voice than to observe the letters they write. If you're struggling to truly know them, or how they interact with others, have them write a letter.