I personally use tricks like making one character speak in riddles, or in short cryptic answers, or even starting with a tick.
<PERSON> cleared his throat. "She was wrong! I never said that!"
<PERSON> looked annoyed. "Blah, blah, blah..."
Something that works sometimes is to make someone very dull, or too intellectual. Or immature, or overtly sexual. This has to fit in the character.
<PERSON> turned to <PERSON>. "I suppose you know what happened. Were you there?"
"You know very well I'll never participate in your twisted little bedroom games."
The arbiter raised his voice. "That's enough you two."
It is possible to also purposedly mingle the two. An old couple:
<PERSON>, sitting next to <PERSON> her husband, lifted her glass of cherry. "To the new child!"
"And to his parents!"
Or twin boys playing.
"It's mine!"
"No, it's mine!"
"You take the green stick!"
"No, you take the green stick!"
"I don't want the green stick!"
"I don't want it! I want the other one. It's mine!"
In this case, it actually doesn't matter which one is speaking...
The old man and his old wife:
<PERSON> began. 627
"When I was your age, I used to walk to school in the snow."
<PERSON> piped in from the other room "... uphill both ways!"
The gentlemanly teen with an older girl:
"I could not, I mean, I dare not ask this, this simple thing"
<PERSON> smiled brightly. "You dare not ask me for a kiss?"
<PERSON> reddened and his fingers shook. "I dare not."
"Then don't ask..."
A lot is revealed of each character by what they will say and won't say. A seasoned sailor, a gent and a port girl.
"'been at sea so long you'll do."
"How dare you sir? This lady is not--"
"300"
The characters are never speaking to the reader, but to each other. It's up to the reader to follow along. Sometimes the mystery is more interesting than the content.
"<PERSON>, did you bring it?"
"Yeah, Boss"
"Where is it?"
"Trunk."
"And ammo?"
"Yeah, Boss"
The reader wants to be able to associate mentally with the characters. Those that open themselves up more, that make themselves more vulnerable, are more likely to feel real to the audience.
"<PERSON>, I've never felt the way you make me feel before..."
<PERSON> remained silent, his brown eyes fixed on <PERSON>'s.
"Is your heart so cold that you cannot speak to me?"
He clenched his teeth. His hands gripped hers tighter.
"Are you so hurt you cannot speak of love?"
He let go and turned aside, burying his face in his hands.
She held him gently and whispered "Oh, <PERSON>, it's all right..."
You'll notice <PERSON> did not speak, and yet much was said. <PERSON>, the silent one, speaks with gestures and emotion. That's his voice, and his spoken words would only support his overall demeanor.
Each character must find his own voice. 627