An Army Veteran is a passive town role that makes the player holding it kill the first person to target them for a kill during a Night Phase. It is a one-shot ability. After it has been activated once, the player becomes a vanilla townie for the rest of the game.
A common variation, perhaps moreso than the original, is for the Army Veteran to kill the first player that targets them at night for any reason, and not just a killer. Also common is for the Army Vet to have the option of using their one-shot ability actively, meaning that they can choose to target a player for a kill during a Night Phase.
 Paranoid Gun Owner
Perhaps a role used more frequently than the Army Veteran role, a Paranoid Gun Owner will automatically kill anyone that targets them at night for any reason, whether positive or negative. This ability is also retained for the duration of the game, unlike the one-time Army Veteran role. However, the PGO is not given the option to make their ability active in the same way as an Army Veteran might be.
You are Moe Syzlak: Stonecutters Aligned Paranoid Barman
To the naked eye, you are the seemingly mild-mannered owner of a simple Springfield tavern, but – little do the rest of the town know – you are also a prominent member of Springfield’s secret society, The Stonecutters. In these times of trouble you, Krusty, Skinner and Reverend Lovejoy will use this top-secret group to talk to each other whilst remaining off the mafia radar . If anybody targets you for any reason whatsoever during the night phase, then your attitude whilst holding your shotgun is to shoot first and ask questions later, which could result in any figure of any alignment ending up six-feet under. Your group also has a one-time-only ability to call a meeting during the night phase by PMing myself with a request to do so– if and when you do this, then any character who targets a member of your group for whatever reason will be on the receiving end of a quick but painful death, and on this night only you will be immune to roleblockers. You win when all threats to Springfield have been eliminated.
The Army Veteran can afford to be more vocal than, for example, a Cop or a Doctor, since they have protection from one attempted night kill and drawing a scum attacker onto them could even net the town a positive result during a night phase. Once the ability is lost and the player is a vanilla townie, they then have nothing to lose and can continue to play aggressively, perhaps causing scum to think they might be a fully-fledged PGO.
A PGO should be out in front, leading the town, since they cannot be killed at night and will kill anyone that tries to target them at night. An unrelenting, but logical, approach to lynches and discussion will often mean that a PGO can lead the town to a victory.