I think good fic always has, as its underpinnings, good meta (even if it never sees the light of day).
I don't mean meta everyone agrees with or even meta that is entirely reflected in the fic you write; just meta that works for you and that makes you think about the characters and how they function in the story that was originally told about them. Thinking deeply about characters and characterizations, why they do what they do, what drives them within canon, is how you can actually embed yourself in their voices, because you've considered how they think and what the story they exist to tell actually means.
