On-the-nose dialogue is a line where what the character says and what the character means are identical. 'I'm angry that you left me' is information delivery. It is efficient and completely dead, because in life almost nobody says the true thing directly — they attack the dishes, the timing, the tone of voice.
Subtext comes from giving characters a reason not to say it. Give the scene a surface topic that is not the real topic. Two people negotiate over a car and are actually negotiating over whether the marriage continues. The audience does the translation, and that act of translation is the pleasure.
Distinct voice comes from what a character notices and what they refuse to discuss, far more than from vocabulary or slang. A doctor and a mechanic can both say 'it's fine' — the difference is which detail each one reaches for first, and which one they will not look at.
The reliable test: cover the character names and read the dialogue aloud. If you cannot tell who is speaking, the characters do not yet have separate interior lives, and no amount of accent or catchphrase will fix that.