I don't know much about D.I.D. but here's what I do know:
-Most people get it after a traumatic experience at a young age
-Some people can have multiple identities
-Sometimes only a certain identities know about the other
-The identities are mostly completely different people ex: One of the identities can play the bass but the other can't.
Try to find out how many identities the character has.
Is it the character that is speaking in 3rd person? If so, that means that your character is aware of the other identities. Meaning friends/family of that character may also know about too.
If the case is that the other identities know about each other, the experience may be different.
For instance- The identities could fight with each other or have a full on conversation.
Maybe your character could have a trigger to their disorder kind of like PTSD.
So then it may be easier to switch into different identities.
That's all I have. Hope it helps, if not could you give me more info. about your character and the setting? 