I have thought a lot about writing something about someone with DID. What I am wondering is, when writing dialogue and such for this character, do I stick to the original name, or do I write the name of the personality or whatever that is talking?

Assuming you're wanting to write multiple personalities they would each have a different look, manner of speaking, character name etc.

Tara often transitions into one of her alternate personalities: wild and flirty teenager T; 1950s style housewife Alice; and male, loud, beer-drinking Vietnam vet Buck. A fourth personality, Gimme, is introduced later in the first season. Wiki

Pilot for United States Of Tara - Diablo Cody - below:

http://www.zen134237.zen.co.uk/United_States_of_Tara,_The_1x01_-_Pilot.pdf

Hope this helps.

Wow, that's actually really helpful, thanks! It was actually watching United States of Tara on Netflix that made me want to write about someone with DID.

Currently writing a screenplay with a character who has 5 personalities.  The first time a personality appears I  described them. In one scene, 4 of them have a conversation.  I wrote the dialogue with the name of the main personality and then who they are at the moment in a parenthetical.  Gregory (as Nigel), Gregory (as Cedric). Wasn't sure about Gregory (as Gregory).  That's how I'm handling it.  Would love suggestions on how it should be done.

Quoted from DSM Currently writing a screenplay with a character who has 5 personalities.  The first time a personality appears I  described them. In one scene, 4 of them have a conversation.  I wrote the dialogue with the name of the main personality and then who they are at the moment in a parenthetical.  Gregory (as Nigel), Gregory (as Cedric). Wasn't sure about Gregory (as Gregory).  That's how I'm handling it.  Would love suggestions on how it should be done.

I like that approach. Don't think you need the Gregory as Gregory though.

Quoted from eldave1 I like that approach. Don't think you need the Gregory as Gregory though.

This is what part of the scene looks like:  Cedric is the only one who had not been introduced.

EXT. DARKNESS - NIGHT

Only Gregory's head and upper torso are illuminated by one dim light on his right side.  Odd sounds echo in the void.

GREGORY
(as Nigel)
Hello.  Is anyone here?  I believe we've got a lot to discuss.

Gregory continues to slowly walk through the darkness.

GREGORY
(as Nigel)
I want some answers.  What the hell is Michael doing here?

The light source is now below Gregory.  He speaks with a slight lisp and his facial expressions become highly exaggerated.

GREGORY
(as Cedric)
Don't get your panties in a wad, mild one.  I thought we needed to try something different.

The light comes from the left side.

GREGORY
(as Bruce)
She still doesn't trust you.  You have got to get her to trust you.

Nigel's light.  

GREGORY
(as Nigel)
I'm tryin'.  I don't seem to be able to have any kind of effect on her.  Gregory's appearance didn't help.

Bruce's light.

GREGORY
(as Bruce)
Gregory.  You were supposed to stay out of this.

Now the light is directly overhead.

GREGORY
(as Gregory)
I couldn't help myself.  She said I was thinkin' somethin' that I wasn't.  I had to tell her she was wrong.

Clear to me.

This might help, too. But I like the idea of using parentheticals as well.

http://screenreads.com/formatting/character_headings/char_name_changes.html

Pity the screenplay for Split, (M. Night Shyamalan) is not available to download.

In Primal Fear the formatting is done this way:

Aaron/Roy smiles.

      Aaron/Roy
You still don't get it. I can't believe it.
Hotshot lawyer like you...

       Aaron
Can I tell you something, Mr Vail...

I’ve yused the / in my script. Two coverages say its okay and it’s in trottier as one of the methods.