Fallout: New Vegas
75 Achievements
1655 XP
60-80h
You'll Know It When It Happens
Complete You'll Know It When It Happens.
20 XP
3.0%
How to unlock the You'll Know It When It Happens achievement in Fallout: New Vegas - Definitive Guide
This is a mission you get during the "For the Republic" mission line for the NCR, while president Kimble visits Hoover Dam. While there may be a couple of ways of completing this, here is the method I used.
Once you are talking to Ranger Grant, ask all question you can of him, make sure you get his itinerary. Then, access the computer terminal next to him(requires science 50), to determine some one has hacked into the system. Now talk to ranger Grant initiate the president event, and while its flying in head to the ladder behind Ranger Grant that leads to the top of The visitor center, where the copter will land. Once it has landed, an engineer will begin fiddling with the plane. examine the plane, and with an explosives or repair of 65 or higher, you will find a bomb. Quickly go tell Ranger Grant, and the speech will be ended(thank god, it is the most boring speech in all of the post apocalypse), Kimble will be hustled to his copter, and the achievement will pop.
Note that there are a lot of "waypoints" on this mission, I imagine there are other methods to complete this, such as possibly catching the sniper, but you would probably still need to remove the bomb anyways, so this is probably the shortest way.
Edit: This can also be done during the Yes Man storyline if the NCR are friendly towards you. Thanks Radiant Viper for the hint!
Edit... This being a Fallout game, there are a number of ways of completing this mission. So I have posted some alternate solutions that have been given by people in the comments section...
Edit:(taken directly from comment, thanks VoodooAK !!)
You can also go to the closest tower behind the stage area that President Kimble will speak at. From the ground you can see a NCR Ranger posted there. Go up to the top and join the Ranger, after a bit into Kimble's speech another NCR Ranger will come up through the hatch and will be shown as an enemy (he attacked me on sight). Kill the enemy Ranger (presumably spy for the Legion), then return to Grant and tell him about the sniper. He will end Kimble's speech, but first a person will run up and try to kill Kimble but is stopped by the Rangers. The 2nd assassin actually killed a Ranger when I played.
Edit(11/17/10): Another solution from BearsinGames...
I went into the storage closet under the stairs up to the roof, and there was a pool of blood. This required a Perception above 6 to analyze, and gives you the result that someone in an Engineer suit is going to attack the President. During the middle of his speech, a third engineer joins the crowd. If you talk to him and insist he comes with you, he attacks you. After killing him, the President escapes safely with no further interactions needed.
This quest is special in that it is given in the NCR, Mr. House, and Wildcard quest lines. For Mr. House and the Wildcard, the quest may not appear if you are not on good terms with the NCR. In either case, the main goal is to prevent the three assassination attempts against the President. There is a bomb on the vertibird, a sniper in the tower behind the president, and a fake mechanic with a knife. See a detailed walk through here.
This is a main quest mission, only available if you are not on the Caesar's Legion ending path, and if your Reputation with the NCR is good. If your Reputation with NCR is bad, this mission will be skipped entirely denying you the opportunity to get this trophy. This trophy and Arizona Killer are mutually exclusive.
"You'll Know It When It Happens" will occur as part of either "Wild Card: Finishing Touches", "For the Republic Part 2" or "The House Always Wins VI". The mission is to prevent the assassination of President Kimball while he's making his speech at Hoover Dam, depicting the opposite side of "Arizona Killer". This mission is easiest if your Reputation with NCR is Liked or better, making Ranger Grant entirely cooperative with your efforts and allowing you unrestricted access to the entire area.
There are two main threats. There is a bomb on the President's vertibird (Explosives 50) to remove. A nearby engineer on the landing pad has a redundant detonator that with a high Sneak skill you can pickpocket. Bringing either one to Ranger Grant's will attention will cause him to cancel the speech and get the President out of there.
The second threat is a sniper in NCR Ranger garb that climbs the guard tower behind the stage. Depending on the timing of your initial search, the sniper may have time to still kill the President if you don't deal with him before you contact Grant. The sniper will kill the other NCR guard on the tower and then shoot the President shortly after this. If you have a sniper rifle, you can take out the assassin from afar (make sure you wait until after he kills the NCR guard, otherwise you look like the assassin). A safer approach would be to climb the tower itself. Once on top, if the NCR guard is still there he will continue his patrol as normal. Wait for the assassin to arrive and make his move, then dispatch him before he shoots the president. If the guard has already been replaced by the assassin, he will instantly challenge you. Insist that you call in the dead guard, and eventually he will turn on you allowing you to kill him.
If the disturbance has drawn attention, the President will be ushered off stage and back to Bear Force One, otherwise he will finish his speech and take off at the end like nothing ever happened. You'll get the trophy once the President is safely away.