Rigor normally appears within the body around two hours after the deceased has passed away with - as we have already mentioned - the facial and upper neck and shoulder muscles first to visibly suffer from its effects. Many Scenes of Crime Officers (SOCO) have reported that upon discovering the deceased that their face might have taken on what looks to be a grimace; this is because the facial muscles have contracted as ATP drains from them.
Once the contracting of all the body's muscles has taken place this state of Rigor - technically referred to as the Rigid Stage - normally lasts anything from eight to twelve hours after which time the body is completely stiff; this fixed state lasts for up to another eighteen hours.
http://www.exploreforensics.co.uk/rigor-mortis-and-lividity.html
Hours 2 to 6
Because the heart no longer pumps blood, gravity begins to pull it to the areas of the body closest to the ground, a process called livor mortis. If the body remains undisturbed long enough (several hours), the parts of the body nearest the ground can develop a reddish-purple discoloration from the accumulating blood. Embalmers sometimes refer to this as the "postmortem stain."
Beginning approximately in the third hour after death, again depending upon numerous factors, chemical changes within the body's cells cause all of the muscles to begin stiffening. Known as rigor mortis, the first muscles affected include the eyelids, jaw and neck. Over the next several hours, rigor mortis spreads upward into the face and down through the chest, abdomen, arms and legs until it reaches the fingers and toes.
Interestingly, the old custom of placing coins on the eyelids of the deceased might have originated from the desire to keep the eyes shut, since rigor mortis affects them soonest. Also, it is not unusual for infants and young children who die not to display rigor mortis, possibly due to their smaller muscle mass.
Hours 7 to 12
Maximum muscle stiffness throughout the body occurs after roughly 12 hours due to rigor mortis, although this will be affected by the decedent's age, physical condition, sex, the air temperature, etc. At this point, the limbs of the deceased are difficult to move or manipulate. The knees and elbows will be slightly flexed, and fingers or toes can appear unusually crooked.
http://dying.about.com/od/thedyingprocess/a/My_Body_Postmortem.htm
Livor mortis (Latin: livor"bluish color," mortis"of death"), postmortem lividity (Latin: postmortem"after death", lividity"black and blue"), or hypostasis (Greek: hupo, meaning "under, beneath"; stasis, meaning "a standing" ) is one of the signs of death. Livor mortis is a settling of the blood in the lower (dependent) portion of the body, causing a purplish red discoloration of the skin: when the heart is no longer agitating the blood, heavy red blood cells sink through the serum by action of gravity. Intensity of color depends upon the amount of reduced hemoglobin in the blood. This discoloration does not occur in the areas of the body that are in contact with the ground or another object, as the capillaries are compressed. As the vessel wall become permeable due to decomposition, blood leaks through them and stains the tissue. This is the reason for fixation of hypostasis.
Coroners can use the presence or absence of livor mortis as a means of determining an approximate time of death. The presence of livor mortis is an indication of when it would be futile to begin CPR, or when it is ineffective to continue if it is in progress.
It can also be used by forensic investigators to determine whether or not a body has been moved (for instance, if the body is found lying face down but the pooling is present on the deceased's back, investigators can determine that the body was originally positioned face up).
Read more:
http://www.deathreference.com/knowledge/Livor_mortis.html#ixzz35OfW5S6W