Normally in a dream state we are also in a sleep induced paralytic state. If that does not occur for whatever reason, our brains still tell the nerves what to signal to the muscles…so we might do something less dramatic like flail around a bit in bed, or talk, or more dramatically walk or even, as some people on Ambien have done, drive! All that has happened is that the brain has not stopped sending signals through the motor nerves to the muscles. We are not always that concious even fully awake of our movement: I may think I will get up and go to the kitchen, but of course do not think at all about the mechanics or where to place a foot, etc. So if I dream about going to the kitchen, my brain can direct my muscles without my full awareness. Biological, not.philosophical.
I was a sleep walker as a child and still talk in my sleep. My son talks in his sleep also. As a kid I once thought I was in a swim meet while asleep and was flailing and kicking, per Mom.