When I am reading a work of fiction, I hear the voices of the character in my mind. No, I don’t hear voices literally. I imagine I hear them. That’s quite different. But, when I hear each of them as I’m reading, they all sound different. I hear them that way because the author did such a good job with creating a voice for each individual character that they all sound different.
So how do we make our fictional characters sound different? It comes down to choice of words and how those words are said. I think body language/facial expression also plays a part in that it helps with how the words are intended when spoken on the page in your dialogue. Think of their character too. Is how they speak out of character for them. If so, why?
Look at the different ways to say something.
“You heard that? Really? I didn’t think I was that loud,” said Bill.
“Hey ya. Serious now, ya’ll hear that? How? Not like I was ‘at loud or nothin,” said Bill.
Play around with your words. See your character in your mind. Imagine them talking. How are they saying what their saying?