This article is from the Dissociation FAQ, by Discord (firstname.lastname@example.org) with numerous contributions by others.
Oh, sorry. This differs so much from multiple system to multiple
system that I don't know where to start.
For some, each alter takes turns experiencing and performing
actions and talking and so forth, sometimes without the others knowing
that it is happening. For some, the alters can jointly run
the body, either in tandem (picture two or more alters sitting
around in a control room, discussing rapidly what to do/say and
sharing the feelings), or by partially merging/overlaying.
For those who have alters who are co-conscious, the degree of consciousness
experienced may vary by alter, by situation, or just by whim. The alters
sometimes may choose how much they feel/decide/interact.
The alters not currently helping run things may or may not retain
memories, emotions/feelings, and so forth. They further may or
may not have any access at all to such, either vicariously (like
watching a video) or more personally (accessing the memories and
feeling as if it DID happen to them) or something in between.
They might talk to one another, they might not.