Life After Death is a glitch present in Halo 3's Campaign. If an NPC is betrayed immediately before a cutscene that depicts their departure or death, the NPC will still be alive and in the level after the cutscene. The glitch was named for its first application: "preventing" the death of Sergeant Johnson.
This glitch does not affect the storyline.
Start the level at Rally Point Bravo. Kill the Arbiter and move his Carbine away from him. Make sure Arbiter picks up his Energy Sword.
If the Thunderstorm skull is not turned on, pick up a Deployable Cover. You can pick up a deployable cover in the room with the eating Brutes; the Brute Minors will usually drop the equipment. When you have reached the Cartographer room, melee the Arbiter to the activation panel and position him so that when you headshot him, he will fall to the lower level. Then headshot him and activate the Cartographer. After the cutscene, you should see two Arbiters; the Arbiter you killed will fall to the lower level and rise while the newly spawned and unkillable Arbiter will hijack the Banshee as scripted and will eventually be removed from the level. Go downstairs, go to the lobby which precedes the overlook of the extraction area, and use the deployable cover to block the door after the Brute Chieftain has left the lobby. Arbiter will usually kill the Brute Chieftain and then he will attack the Jump Pack Brutes. You can temporarily disable the deployable cover (without destroying it) by draining the shield to exit the lobby while Arbiter engages the Jump Pack Brutes. Consider deploying Auto-Turrets to the lower level (to assist Arbiter or to kill most of the Jump Pack Brutes) before going down.
If the Thunderstorm skull is turned on, Deployable Covers will not be available and after retaining the Arbiter, he will only be able to kill the Brute Chieftain, Brute Stalkers, and Jackals. He will only be able to remain in the lobby (he will run back into the lobby if you try to move him) until you have killed some of the Jump Pack Brutes. Then he can be moved out of the lobby. You can move him to the cutscene area, knock him out, and then trigger the cutscene so that he appears in the cutscene.
Halo (Thel 'Vadam)
The first step requires you to be able to kill Thel 'Vadam right before you enter the room where the battle with 343 Guilty Spark is fought. In order to achieve this, you will have to perform the Spartan Laser Glitch, and it is best to try it on Easy.
Once your Spartan Laser has been glitched into having infinite ammo, go to the door immediately before the fight with 343 Guilty Spark, but don't enter. Shoot Thel with the Spartan Laser until he dies. Quickly after that, go through the door and initiate the cutscene. Once you finish the fight with 343 Guilty Spark, go outside and there will be two Thels. One will be standing, looking at you, and another will already be attacking the awaiting Flood; the latter has the waypoint above his head. Once near the end of the hallway, both Thels will stay back.
Halo (Sergeant Johnson)
On the level Halo, when you arrive at the entrance to the Control Room, shoot Sergeant Johnson in the head to make him fall for 5 seconds. Enter the Control Room while Johnson is down. If done correctly, he will still be alive after the cutscene. However, after 3 seconds, a Flood Infection Form will jump onto him. If the player is able to save Johnson from the Infection Forms, kill Johnson twice to get his Spartan Laser with unlimited charge.
On higher difficulties, the Infection Form will vanish, unable to infect Johnson. He sometimes disappears as soon as you leave the Control Room's entrance door, though there are at least two recorded instances where a player brought Johnson all the way to the end of the level.
It is known that, under normal circumstances, the affected characters are removed from the level after the affected cutscenes; on The Ark, Thel 'Vadam departs in a Banshee, and on Halo, Sergeant Johnson dies. The glitch prevents them from being removed from the level.
This glitch prevents the removal of a NPC because when alive they have an instance of their name or something relevant to their name. But when this NPC dies their instance is no longer their name, it becomes something like Dead or Ragdoll. The game's code is set to remove the instance of the characters name, not Dead or Ragdoll. Thus allowing players to glitch NPC's past cutscenes or loading sequences.
When proceeding through these levels normally, important NPCs, such as Johnson and Thel 'Vadam, are deleted from and re-added to the levels several times—each time an important NPC gets separated from the player, they are removed. Killing such an NPC will prevent their removal. When the game re-adds him later, it does not check to see if the previous NPC was successfully removed.
This, then, is exactly the same glitch as the Three Johnsons glitch and the Two Arbiters Glitch.