Interruptions can also create more variety in your dialogue rhythm. It adds a nice change of pace without being too much of a hitch in the reading flow.
Now, don’t go overboard and have people interrupt each other all the time (even though we know that in real life, that can happen). Moderation is the key, as with any writing style.
Finishing a sentence:
This is a fun type of interruption, when the other character finishes the person’s sentence for them.
“This is a private Christian school, kiddo. You sure you’re supposed to be saying that kinda word around here?” Joel asked.
Bradley jerked his head around, his eyes rapidly scanning the perimeter as if they’d just come under enemy fire. “N-no. I ain’t supposed to. Good thing my teacher’s not—”
“Right behind you, Bradley?”
--From A Soldier’s Promise by Cheryl Wyatt
Cutting someone off:
This is a great device to heighten tension and conflict in a dialogue. Again, use this judiciously or else your character will sound like a jerk/witch for always cutting the other person off.
She snorted. “Some things never change. I made a mistake calling you, Luc. You’re too much like the old man to see reason.”
Ouch, that stung. “I’m sor—”
“Consider yourself warned. My family will fight you Trahans.”
--From Bayou Justice by Robin Caroll
When someone is interrupted, use an em-dash, like this: —. It’s a long dash, the length of the letter m on an old-fashioned typewriter. The short dash is called an en-dash (the length of the letter n), and that should not be used to indicate an interruption. If you don’t use an em-dash, use two short dashes like this: --
Do not use ellipses to indicate someone is being interrupted. Ellipses (three periods like this: ...) are used to indicate when someone’s voice is trailing off, but no one is speaking to fill in the gap of silence.
”It’s him, Ma. We know our own pa!”
“It’s ... it’s ...” Sophie struggled to let go of the wild surge of hope that was building in her.
--From Petticoat Ranch by Mary Connealy
Look at your dialogue passages. Can you add an interruption to vary the pace or increase tension?