Cover the character names on any page of dialogue and read it aloud. If you cannot tell who is speaking, the characters do not yet have separate interior lives. Almost every writer fails this test on the first draft, including working ones — the difference is that they run it.
Voice is not vocabulary. Slang, accents and catchphrases are the cosmetics of voice, and they wear off fast; a character who says 'buddy' every ten lines is not distinct, just marked. Real voice comes from perception and avoidance: what this person notices first in a room, and what they will not discuss.
Rhythm carries as much as content. Some characters answer in full sentences; some answer with the last three words of your sentence; some never answer, they redirect. Give each principal a default move and let stress break it — a character whose rhythm changes under pressure reads as alive.
Status is the third lever. In every scene, someone is trying to raise or lower their standing. Two characters written at identical status sound identical, because nobody is manoeuvring.