In my experience with what I consider to be my alters, different ones will step in for me and describe what I am feeling about certain things and will even be assertive if they seem to think that it is needed to get the point accross to someone who I normally have difficulty expressing myself to. I live with one of my brother's who is less thn sympathetic most of the time and demanding as well as other things that don't seem right to me and I can hardly ever talk to him about problems without him trying to end the conversation because he doesn't care or doesn't want to hear it. When I feel unfairly treated by this behavior from my brother, one that I sometimes refer to as "persona G" will come out and defend me in a very calm but, firm way without backing down until I feel at ease or relieved. Others will speak up for me too in other situations and to different people when I can not.
That's how it is sometimes for me anyway.