on Writers Helping Writers:

Vocabulary and the way a character speaks are the outer layer of character voice—the icing on the cake. Instead of trying to build character voice from the outside in, get under the character’s skin by revealing how they experience and interpret the story world from the inside out.

Character voice bubbles up organically when every aspect of the story is seen through a character’s-eye view of priorities, perspectives, and agendas.It’s less like cobbling together a latticework of characters, setting, and events than it is establishing a running commentary on how the character views everything caught in that web.

“Running commentary” may sound like something suited for first-person or deep third point of view. In fact, continually inflecting the story with a character’s personal concerns is a fit for any point of view whose narrator is also a character. It’s a seamless way to write. The character voice—with all its attendant observations, judgments, opinions, prejudices, preferences, thoughts, and emotions—effectively becomes your framework for worldbuilding.

The idea of character voice often brings to mind a character’s favorite words and phrases—for example, whether a character calls something neat, cool, lit, or dope. That’s coming at character voice from the outside in. To build character voice from the inside out, start with what the character observes in the first place.