Say you have somebody with split personalities and they speak to them and we the audience hear this. How do you write that?

I was thinking of using (V.O.) but it's not a voice over and Off Camera and Off Screen also don't work as they make it seem like another person is there but they aren't.

Maybe you could give different names to the different personalities (GOOD JACK / BAD JACK, CALM JACK/ANGRY JACK, etc). If you do, you can use something like this:

SMEAGOL stares into the MURKY depths of the WATER.

               The SURFACE RIPPLES as the face of GOLLUM appears as SMEAGOL'S
               REFLECTION.

                                     GOLLUM
                         What is it saying, my Precious, my
                         love? Is Smeagol losing his nerve??

                                     SMEAGOL
                         No! Not! Never!! Smeagol hates nasty
                         Hobbitses! Smeagol wants to see them
                         dead!

                                     GOLLUM
                         And we will -- Smeagol did it once...
                              (sly)
                         ...He can do it again.

Another way is to use normal dialogue with your character, and V.O. for his other personality.

I'd go in more depth but if I did I'd have to give away the secret character added into my MM screenplay.

If I do it the Gollum and Smeagol way can I trust people to understand that's what I'm doing without having a character say that or should I say that?

We've actually been discussing this very topic on this thread called How do I...: http://www.simplyscripts.net/cgi-bin/Blah/Blah.pl?b-screenwrite/m-1143402234/

I see how you could mistake that but it's not a reflection, it's a whole other person. Hard to explain without giving away the idea but trust me it's not the same question, my character isn't talking to himself or a reflection of himself which would kind of be a cliche movie moment.

I guess we'll see how the scene works when it's done.

I understood that, but that thread dealt with more than just a reflection. One of the last responses I gave had very little to do with a reflection but a whole other person such as you have here.

In The Recluse i just had Constantine have a VO and then the actually Constantine would have conversations with his VO. This would be obvious that he has at least something wrong with him which could eventually lead to the outcome of the splut personality.

I read the entire post before I replied, I'm almost certain that my problem and that entire thread are different. Maybe I worded it wrong to begin with but it's hard not to give away my secret character otherwise and I like having a little surprise when I write something.

I'll take the advice given in that thread and try to use it in a similar way.