Dead bodies do not move nor sit up. The dead body might make some involuntary movements as it is no more moved by the soul in it. It will make some movements that happen as the last events of whatever processes that were going on at the time of death. The tiny movements of the body occur due to the air that is coming out of the lungs, the movement of gas in the intestines, and the electrical current that moves from the heart. The muscles stop relaxing and contracting. The muscles become tight, and that might create small movements at the time of rigor mortis. When the body starts decomposing, the gas that is formed might make noises inside,which might appear as if the body is making some sound and talking something. Death makes the muscles contract as their last movement, and the diaphragm will push the air out of the lungs. This will make the body to make sound as if it is moaning.
The rigor mortis process also starts to happen about two to six hours after death. Though the body is dead, there are certain things in the body which are alive. The skin will be alive around one day after the death of the person. The bacteria that are living in the intestines and pancreas start decomposing the body by spreading to other organs and begin the process of destroying the host. As the organisms start decomposing the body, the color of the body slowly changes. The bacteria create a gas by decomposition which creates a very bad smell.
The air in the lungs when coming out might make the face and mouth also to make some little movements. It is not regular to see dead bodies move. But it might happen sometimes due to the above-mentioned reasons only for a short period of time after death. The further changes that might occur due to the bacterial decomposition might happen after a very long time.