Just some interesting facts about MPD:
1: The personalities can do different things at once, for example; one can talk in the telephone while the other is reading a newspaper at the same time, without changing focus. (one is controlling the eyes and one is controlling the ears and the mouth)
2: The personalities can have differences in most things, like different allergic reactions, one can be allergic to cigarettes, while another one is smoking every day.
3: No personality feels secundary to the "main" personality (at least not from what I've found) even though they probably cannot say exactly when they were born, they are just as individual as the "main" personality. If you ask one of the personality which one is the main one, they probably would get confused or say that it's a certain personality because "she was born in the body".
4: Each personality has it's own characteristics (this you've probably heard before) and no personality has to be violent in any way (as is the misunderstanding that every MPD is dangerous). However, there are some roles that the personalities have, one role can be to protect and handle difficult situations where the "main" has failed to do so herself. Thus there also might be a agressive type in one of the personalities (or in many), but not necessarily meaning that the personality is dangerous because of it.
5: The personalities can be of different age than the "main" one. the age is usually when the personality was formed and often around that age the "main" had a traumatic experiance.
6: When another personality takes control then the "main" feels like she is looking at a theatre, seeing what happens but are unable to control it, they may describe it as looking out of a window. Being in the background.
7: The personalities can sleep, and have their own places to go to in the "main"'s mind, from what I heard, they usually sleep when not being active.
Well, that's about it...