When writing a screenplay, how do you indiacte when someone is cutting somebody else's diaglogue off? Characters are always interupting one another's diaglogue, and I want to know how to write it. I've been going like--
--Hey, is that--
--that, but I'm not sure if that's right. Help?!?

I'd try something like:

CHARACTER #1
Hey, is that--

CHARACTER #2
(interrupting)
Oh, don't start that--

CHARACTER #3
(interrupting CHARACTER #2)
Do you mind? I'm trying to listen!

I don't know if that's the "official" way to do it, but you might try it for purposes of Script Frenzy, at least. You can always go back and fix it later. :)

Aibhinn

---
Find the next word. Write it down. Repeat. Repeat. Repeat. --Neil Gaiman

That's pretty much it - just an M dash at the end of the line where they get cut off.

You can use the parenthetical (interrupting) for the next character's line, but once you establish that's how you're using the dash in conversation, it shouldn't be necessary (maybe three instances).

For characters that trail off or mumble away, ending with an ellipsis is common.

NaNoWriMo & ScriptFrenzy Staff

When someone's dialogue just trails off it's indicated by elipses....

JOE
It's not like I want to go.....

Interruption is indicated by dashes --

JOE
It's not like I want to go to --

MARY
-- oh gimme a break! you know you're
dying to see --

JOE
-- don't bring up her name again!

So there's no need to add a parenthetical unless it would be really confusing without the direction.

Generally it's adviseable to avoid parenthetical instructions, aka "wrylies", i.e.

JOE
(wryly)
No kidding?

They're frowned upon an example of the writer 'directing'. Of course every script has parentheticals, but keep them to a minimum, use only when necessary.