At first it can be scary. My friend has several alters, 2 of whom have a tendency to be violent. So when he shifts, it's clear, the face and voice change. The female alters are probably the most noticeable when they need time. Mannerisms of the body change, heck he even walks differently. With other alters, the change is more subtle and I have to ask him his name. I recommended he get help for it because he was initially complaining to me about not remembering 6-12 hours of the previous day, so he was unaware. I think the client is the conduit for the therapists knowledge, but a therapist with experience will ensure that the primary client is kept informed. With consent this can involve appropriate video and audiotaping in session, as well as something called "conferencing" where everybody agrees to have a period of gathering and communication.