I can tell you from a former sleepwalker's vantage point, the talking is about what they are dreaming about. Someone once told me I couldn't have been dreaming if I was walking around, but in all instances, when my sister told me what I was saying, I could relate it directly to a dream I was having. Once I got up and walked into the living room, picked up the phone and started to dial (I was 8). My father asked me what I was doing, and I said "I have to call, I have to call". In my dream I was trying to call my teacher about something, but when I got her on the phone she yelled at me. So when my father asked, "Who do you have to call?" he said I got a really strange look on my face, then I said, "Um, no one" and went back to bed. That was probably the point in my dream just after my teacher yelled at me!