With the Mister Sandman perk, when a character is sneaking, the option to silently kill any human or ghoul while they're sleeping is available. XP from killing the target is earned. Mister Sandman kills give a flat 50 XP per kill.
Mister Sandman is performed in sneak mode and by choosing "pickpocket" on an unfortunate sleeping victim. The game pauses and brings up a menu with the options to "murder" or "pickpocket". Choose the former to activate the perk. When used, a short murder animation follows with the character cautiously looking around and slitting the victim's throat, accompanied by a cartoony jingle.
Possession of a knife or other bladed weapon is not necessary to perform a Mister Sandman kill, even though the kill animation shows the deed performed with a combat knife.
No Karma will be lost or gained from Mister Sandman kills in Fallout 3 (Karma is lost for killing a person while detected). However, in Fallout: New Vegas good Karma will be gained by killing Benny with the perk.
If an unarmed weapon is equipped when a Mister Sandman kill is performed, it will be used during the animation.
Having the Contract Killer or Lawbringer perk(s) also active, killing ordinary characters with this perk will not make the victim drop an ear or a finger upon death (unless they are killed while detected.)
Mister Sandman kills do not affect Reputation as long as the character is [HIDDEN] when initiating the kill sequence.
It is possible to gain unlimited amounts of XP from sleeping children and anyone flagged as essential. When a child is chosen as the victim, the animation is played and the XP is gained, but the child does not actually die, allowing them to be immediately "murdered" again. This can cause a lot of side glitches however from freezing to spawning in other locations without warning.
Despite the fact that the icon image shows a kitchen knife the murders are done with a combat knife in Fallout 3.
If the victim gets up during the animation, they will still die when it finishes.
During the murder animation, the killer stands up. In that instant, they are no longer sneaking and a normally already hostile character (e.g., a Raider, as opposed to a Megaton Settler) immediately detects them and awakens. The victim gets up and acts normally (e.g., cry out and shoot at the murderer) but shortly after the murder animation completes (even though they are no longer in bed) they suddenly lose all health and fall down dead.
pcxbox360 It is possible that while murdering the victim, they will wake, stand up and go hostile, however, they will still die after the murdering animation is complete. It seems that when the character being killed starts attacking the murderer, no damage will actually be dealt by them. [verified]
ps3xbox360pc In Fallout: New Vegas ordinary characters killed by this perk may fall through the floor making it difficult to loot them before they fall.[verified]
pcxbox360 For unknown reasons, in some cases, the Mister Sandman animation will be replaced with with the going-to-bed animation. As soon as the player character begins to lie in the bed, the NPC will begin to get out of the bed simultaneously. Upon lying down fully, the NPC will die and the PC will immediately return to the previous crouching position.[verified]