They are pre-set companions that will act like any other companion in every RPG ever. I called that a week ago :p It's kinda schizophrenic to role-play all the characters when they talk to each other, but whatever floats your boat, I suppose. I'm pretty positive we'll reach Mod Support, so I'm guessing someone will make a mod that will allow you to create your own party, old-school style.
I just didn't like the fact that AI companions won't act against us, ever. At least that's what I understood. They won't ever leave or attack us no matter how appalling or insulting we are to them. EDIT: They haven't mentioned how many companions there are, actually. So if it's only 3, then them not leaving would be a gameplay choice and not a story one.
This can be a good or a bad thing, depending on the way the story is set up. It sure would make for a more focused narrative that can intertwine beautifully with these four characters, this is somewhat comparable to Mask of the Betrayer. Okku and One-of-Many felt a bit like baggage compared to the HUGE impact Safiya, Gann and Kaelyn had on the story. I always thought that the "canon" idea for the party was You, Safiya, Gann and Kaelyn.