The Preacher is standing on the second floor of a building in an alleyway west of the Ranger compound entrance in Seward Square. From there he preaches mad "logic" to anyone that can hear him. He rants about the apocalypse, the sun, his headache, and an evil worm that he plans to blow up. The alley he overlooks has been rigged with frag mines and mini nukes that he detonates if you try to get close to him. There is a hostage (another wastelander ) on one end of the alley you can talk to about the booby-trapped alley and the Preacher.
You can talk to the hostage and, with a Charisma 1 check, convince him to confront the Preacher for negative Karma. This will result in him running towards the Preacher but being blown up by the mini nukes and the Preacher dying as well.
Speaking to the hostage once the Preacher is dead (assuming you killed him before he could set off the explosion event) will result in no dialogue options but the hostage will comment on how quiet it is now. He will still stay in the same place however, and there is no way to "release" the hostage.
xbox360 The player may find the Preacher dead and the hostage missing, but all of the explosives still in place. [verified]
pcps3 The Preacher will finish his sentence even after you kill him. [verified]
xbox360 There is often a possibility that the Preacher may not be killed by the explosion (usually because the player has managed to disarm the mines while wearing a Stealth Boy). This may stop any interaction with either the preacher or hostage, and the message saying that they are fleeing will appear even though they are walking normally. [verified]