You Gotta Shoot 'Em in the Head

You Gotta Shoot 'Em in the Head is a side quest in Fallout 3 as well as an achievement/trophy.

Quick walkthrough

 * 1) Talk to Mister Crowley in Underworld.
 * 2) Collect the keys from his former associates.
 * 3) Give the keys to Crowley.
 * 4) Go to Fort Constantine and get the T-51b power armor yourself.

Detailed walkthrough
Allistair Tenpenny once hired five mercenaries; Mister Crowley, Tara Fields, Dave, Dukov, and Jeff Strayer; to find a rare weapon in Fort Constantine. In order to get a bigger share of the reward, Dukov trapped Mister Crowley (a ghoul) in a room with feral ghouls. Crowley, who survived despite what everyone else believed, is now a resident of Underworld. Tara died in Fort Constantine and the three remaining mercenaries escaped and completed the mission.

Starting the quest
Talk to Mister Crowley in the Ninth Circle or at Carol's Place in Underworld. He gives you the task of finding and killing Dave (at the Republic of Dave), Ted Strayer (in Rivet City), Dukov (at Dukov's place), and Allistair Tenpenny (at Tenpenny Tower). He says they are ghoul-haters, and he wants them each killed with a shot to the head (as zombies are killed in fiction). For that purpose, he will give the player a sniper rifle and five .308 caliber rounds. Crowley offers 100 caps per death by headshot with the sniper rifle as payment. He says that "word will get around" if Tenpenny is killed, but he wants proof for the others: "something personal... like a key or a ring."
 * Crowley will only pay 25 caps for any killing method other than a head shot, but you can lie about the act and get the full price by passing a somewhat difficult speech check. This does not apply to Tenpenny; he must be shot in the head by you to get the full reward, since word of his death would spread rather quickly.
 * Killing anybody grants you negative Karma although killing Tenpenny grants you positive Karma.

Crowley's true motives
Bribing or passing a speech check with any of the named non-player characters in Underworld – Ahzrukhal, Carol, Doctor Barrows, Winthrop, Patchwork, Tulip, or Quinn – will raise doubts about the targets' bigotry. Under questioning, Crowley will admit that he actually wants a special key from each target except Tenpenny. He still wants Tenpenny killed with a shot to the head. Instead of 100 caps per headshot, he will sweeten the deal by offering 100 caps up front plus an additional 100 caps for each key. Mister Crowley's objective is the suit of T-51b power armor still stored at Fort Constantine. You can learn about Fort Constantine and the power armor from Crowley (with a speech check). Other than the additional reward, the conditions of the quest do not change by learning this.

Tenpenny's counter offer
Tell Allistair Tenpenny that Crowley sent you to kill him. Tenpenny will offer 300 caps if you agree to kill Crowley instead.
 * Tenpenny pays 100 caps up front, so it is possible to double-cross him and just pocket the caps.
 * Killing Crowley and returning to Tenpenny for the reward ends the quest.

Getting the keys
The keys can be obtained by speech checks, pickpocketing, or by looting the key from the owner's corpse. They may be collected in any order and can be delivered to Crowley one at a time or all at once.

Jeff Strayer is dead, but his son, Ted Strayer, is now in possession of his father's key. Ted lives in Rivet City.
 * Ted Strayer
 * Ted can be paid for his key.
 * He can be convinced with a speech check.
 * He can be threatened (Strength 6+ or Toughness perk)

Dukov can be found at Dukov's Place.
 * Dukov
 * Dukov can be paid for his key.
 * The Black Widow perk can be used to convince him to give up the key.
 * Male characters with the Lady Killer perk can persuade Fantasia to get the key.
 * Cherry will retrieve the key if the player passes a speech check and agrees to take her to Rivet City. The speech check option appears only once. If you reject taking her to Rivet City you won't get the chance a second time.
 * With a successful speech check, Dukov can be intimidated into giving up the key. However, this speech check is very hard to pass even with a Speech skill of 100. Saving before talking to Dukov is strongly suggested if you choose this method.

Dave can be found at The Republic of Dave.
 * Dave
 * With a speech success, Dave will accept the player as a "Wasteland Ambassador" and present his key as a gift. He will also give you the key if you pass another speech check that is given if you failed the ambassador one.
 * After completing the unmarked quest Election Day in favor of either Rosie or Bob, you can take the key from his body in Old Olney.

Quick Reward

 * Deliver the keys
 * Kill Allistair Tenpenny and give Mister Crowley all three keys to collect your reward. He will then proceed to Fort Constantine to retrieve the armor. He will return to Underworld after a few days (provided he did not get killed in the Wasteland). At this point you can kill him or use reverse-pickpocketing in order to obtain the armor (he will be wearing it). The quest is complete after handing him the keys.

Better Reward

 * Retrieve the armor
 * Proceed to Fort Constantine after obtaining the keys. Fort Constantine is located near the northern edge of the map, east of Raven Rock. When you arrive at the fort you will encounter some Brotherhood of Steel Outcasts fighting protectrons and Mister Gutsies. Go to the small house labeled "CO Quarters", located down the hill from Bomb Storage, and head downstairs. In the basement you will find a dead wastelander by an open safe. The door by the safe requires Strayer's key, and allows you access to the Launch Control Bunker. Continue down to the Bomb Storage. You gain access here with Dukov's key. Inside the Bomb Storage facility, you will find Tara's body on the opposite of the two Very Hard locked doors. Go to the nearby door, open it with Dave's key and disable the stasis field to acquire the T-51b power armor and T-51b power helmet.
 * There is a warhead storage key on Tara's body. It unlocks a door that's just up a set of steps from here. This leads to a large room where nuclear warheads are stored, and the area is consequently highly radioactive. This key also opens either of the Very Hard locked doors topside that lead to the Bomb Storage facility.
 * If you choose this solution, Mister Crowley can still be found in Carol's Place in Underworld. He does not become hostile, but refuses to interact with the PC anymore, only repeating his line: "You stole what was rightfully mine..." This completes the quest.

Maximize Reward

 * Deliver the keys to Crowley and retrieve the armor yourself
 * Kill Allistair Tenpenny and give Mister Crowley all three keys to collect your reward. Then follow Crowley on his long journey to Fort Constantine. He will require a lot of protection from enemies in the Wasteland or else he will die along the way. Crowley will become hostile upon reaching Fort Constantine, at which point you can kill him to retrieve the keys.
 * If you don't want to follow Crowley, you can give him the keys and pickpocket them back. You can also go to Fort Constantine first if you've been there already, open all the doors but don't grab the armor, and give Crowley the keys to get the reward. Then fast travel back and grab the armor. Or you can simply follow him into the lobby of the museum and kill him there. You will not lose any Karma.

Behind the scenes

 * The title is a reference to the rule in zombie fiction that zombies must be killed by destroying their brain. Mr. Crowley himself refers to these legends.

Bugs

 * After completion of Operation: Anchorage, when picking up the winterized T-51b power armor and equipping it, the trophy/achievement for this quest may suddenly become unlocked. If you drop it from your inventory and do not have a convenient game save, leave it where you dropped it and retrieve it after completing the quest.
 * After accepting Allistair's offer and killing Mr. Crowley, Allistair may not have a speech option to finish the quest. If this happens, you can repeatedly talk to Tenpenny and he will eventually recognize that Crowley is dead and give you your reward. Alternatively, take the T-51b power armor from Fort Constantine and complete the quest. If you do this, you will miss out on the reward for killing Crowley.
 * If you have Bloody Mess and/or the body's head explodes, it will not be registered as a head shot, and you will not be able to receive the full 100 caps for the kill.
 * After accepting Allistair's offer and killing Mr. Crowley, Allistair may give you a reward, but the quest will not end. It will keep telling you to collect your reward from Tenpenny. If you talk to Allistair again, he acts as though you have not even mentioned Crowley yet.
 * Sometimes the option to acquire Dukov's Key through the "Black Widow" perk does not appear in dialogue.
 * The quest will sometimes complete itself when switching to a different quest, granting the achievement and registering that you have killed people that you have never met.
 * If you kill Tenpenny, then kill Mr. Crowley, you will not fail the quest. It will tell you to talk to Tenpenny, and then to talk to Crowley, making it impossible to finish the quest.
 * On his way to the Fort, Crowley may get stuck between Underworld and the Mall. This glitch can sometimes be fixed by reentering the underworld and the Ninth Circle, then exiting the museum to the mall Mr Crowley will be there.
 * It is possible that after completing the quest, Crowley can show up in the Mobile Crawler at Adams Air Force Base.
 * You may be able to tell Crowley you killed Tenpenny but not have the dialogue option of saying you shot Tenpenny in the head, even if you did.
 * Sometimes when you give Crowley multiple keys, then talk to him about his true motives, the speech dialog only allows you to receive non-headshot kills for 25 caps for every key afterwards, even though he promises 100 per key.
 * If you acquire all keys, turn them in to Mr. Crowley to get the reward, and then kill him, you can take his items off of his corpse. However, if you accepted Tenpenny's counter offer, you will be unable to receive the reward from him. When you talk to him, there is no option to tell him that Crowley is dead, but you can do a speech check (if you haven't done it before) and earn another 200 caps.
 * Giving the winterized version of the armor to a follower such as Fawkes may cause the mission to be completed. Doing this will cause dialogue with Crowley as though you have completed the mission by taking the T-51b power armor from him.