How do you write dialogue of one character interrupting another mid-sentence? I’ve seen it as (interrupting) next to the characters name, I’ve seen it below the name and I’ve seen it in the dialogue itself.
You have several choices. Use whichever one works best for the situation.
Truncating the first speaker’s line with double dashes (or an ellipsis) is common:
I simply can’t tell you how honored we are --
Yes. We’re swollen with honor.
A parenthetical (interrupting) may be needed if it’s otherwise unclear that the second speaker is changing topics:
No ship has ever navigated a subatomic fissure that size.
Then we’ll be the first. Ensign, bring us about, engines at fifty...
Plasma fragment! Dead ahead!
It’s also common for action to interrupt dialogue:
The Great Pigeon Army will never be defeated! Our dirty wings shall fill the sky, and our excrement stain the land!
A red laser light -- a sniper’s aim -- glows on Gideon’s feathered chest. His compatriots COO in alarm.
Never more will we beg for the baker’s scraps, those piteous crumbs of...
Gideon’s LIEUTENANT WHISPERS into his ear. Gideon looks down at the dot on his chest. He releases a squirt of white from his tailfeathers.