I’m not sure if this has been asked before, either here or in the Straight Dope archive, but I was just wondering. What causes sleep talking/walking? Why do some people do it and others don’t? How is it even possible for people to get up, walk around (doing activities and acting normally), or hold conversations while they’re asleep?
Sleep and dreams have always interested me, so I hope I can get some good answers. Thank you! 
I don’t know the nitty gritty details but during a certain phase of sleep when dreaming occurs, part of the brain gets disabled, presumably to prevent the dreamer from physically acting out the dream. Under some circumstances the disabling doesn’t quite take. This article discusses it as a disorder.
I have done all of the above but I also suffer from sleep paralysis which is sometimes enough to make you want to to commit suicide some days. With sleep paralysis, you wake up yet you cannot move at all or, more importantly, you can move one fingertip a little bit so you have to struggle very hard to get the whole finger to move and you work your way from there until you can get a whole limb to be semi-functional and then you can use that to get other body parts working even if it means throwing yourself onto the floor to “wake up” the rest of your body. This might take 3 minutes or so with extreme effort.
The brain is supposed to have a very efficient on/off switch that tells it not to act physically for things that are sleep induced and vice-versa but it doesn’t always work correctly. I have sleep walked many times with no memory so that switch is faulty can be faulty in that way as well.
As I understand it there’s a tiny area in the brain that has the function of suppressing movement while you sleep. If it’s damaged or destroyed, you can get up and act in your sleep ( this can be artificially duplicated in animals ) This can be dangerous; I recall reading of a case where a woman tossed her children out of a second floor window because she dreamed her house was on fire.
So to answer the OP; some people can act or talk in their sleep, because the brain tissue that’s supposed to stop them isn’t stopping them in their case.