Thus far, we've tackled every class but one -- the Spy. My personal favorite amongst the classes, the Spy relies far more on cunning and strategy than simple firepower. He can turn invisible, disguise himself as the enemy, electrically sap engineer structures, and instantly kill any enemy with a stab to the back -- in other words, he's the perfect class for cowards and manipulative jerks. And badasses.
To find out how to get the most out of the Spy, and how to easily gank one when confronted, hit the jump.
When playing as him:
-Stick to the road less travelled
If you touch anyone whilst cloaked, your body will glow either red or blue (depending on your team), and the person you bumped into will more than likely kill you very, very quickly. If you bump into anyone whilst disguised, you'll probably irritate the person you bumped into, who will then shoot you on general principle, since (A) you bothered him, (B) there's no friendly fire in roughly 99% of the game servers, and (C) you can never be too careful when it comes to Spies.
To combat this, don't hesitate to take meandering routes to get where you want to get, and don't ever confine yourself to passages where only one person can get through at a time. Running into someone whilst cloaked or disguised is probably one of the worst things a Spy can do to blow his cover, so stick to the walls and be ready to dodge any enemy soldier who is moving in your direction.
While cloaked, also be wary of the fact that some enemies will intentionally walk in a wide, erratic path, solely for the purposes of bumping into Spies. Be very, very careful around these people, and give them as wide a berth as possible.
-Don't overestimate the power of the cloaking device
The cloak is actually pretty slow to activate. I've seen many Spies who assumed that they were fully cloaked once their onscreen hand turned 99% invisible -- this ain't the case. When you turn invisible, you do so with an extremely noticeable puff of smoke (UPDATE: the smoke puff occurs when you put on a disguise, not when you cloak -- still, this strategy otherwise still stands, as the cloak still takes a couple of seconds to fully evenlope your body), which takes about a quarter of a second to dissipate.
Therefore, hitting the cloak button (right click) and the immediately turning around a corner and heading toward the enemy is not a viable strategy. Give yourself a second, then go. This will drain a bit more of your cloak's already-short power supply, but it's more than worth it in the long-term.
Enemies are much more suspicious of people who are facing the wrong direction than ones who are facing the right way, but happen to be moving backwards as if retreating. This strategy works even better if you:
-Get your friends to shoot at you
It is much more believable that a Scout or a Soldier would be retreating from battle when he is being fired upon. If, as a Spy, you have to dodge your own teammates' rockets as you backpedal into the enemy base, the enemies will be much more likely to take you seriously and buy into your disguise.
This is actually win-win-win; your disguise will be more believable, you won't take damage from your own teammates' fire, and other enemies, seeing one of their own teammates being fired upon, will rush to fight the attackers and not spend their time preoccupied with debating whether to shoot you or not.
-Make sure your disguises make sense
A Scout has no place in a flag room. A Sniper has no place underwater. Think about what you would do if you really were the class you are disguised as, and place yourself in that location.
You also need to make sure that there is a plausible reason that you aren't firing at anyone. If you're disguised as a red Heavy Weapons Guy and a blue Engineer is on the other side of a doorway, nobody will buy your disguise since a real HW Guy would have no reason whatsoever not to walk through that doorway and tear the Engineer apart.
Similarly, don't spend too long around people if you're a Medic, because Medics draw a hell of a lot of attention. Enemies will approach you, follow you, and demand healing from you -- and when you don't deliver, they'll know you're full of crap.
Additionally, be aware of the speed changes which accompany all of the disguises; if you switch to a Soldier, you'll walk as slow as a Soldier, which may not be particularly helpful if you really need to rush somewhere. Conversely, if you switch to Scout, you'll look like a Scout, but you won't be as fast as one -- really observant opponents will notice this.
-Don't take on an Engineer and his sentry gun all by your lonesome
This used to be a viable strategy back before players wised up and kept their backs to the wall. Try it now, and the Engineer will either knock off the sap and let the sentry kill you, or he'll shotgun you to death and then knock off the sap.
As said in the Engineer article, a Spy should work cooperatively with someone else; the Spy runs in, saps all the sentry guns, and while the Engineers hurriedly try to destroy the saps, a Demoman or Soldier or HW Guy destroys the disabled sentries. The Spy should stick around to replace saps to the guns, should any of the Engineers succeed in removing them.
Granted, this little suggestion only applies if you're playing against an Engineer intelligent enough to completely barricade himself behind his buildings and not bother chasing you around, thus giving you a chance to backstab him. I'd recommend trying to destroy an Engineer and his stuff once, so as to get a feel for the sort of Engineer he is; if he panics and tries to kill you before knocking the sap off, then he's nowhere near as formidable an opponent as he could be. Go ahead and try to take these guys one-on-one, if you feel confident enough.
-Restock on health at your base
The Spy shouldn't, or rather can't, spend that much time behind enemy lines. Sooner or later, someone will expect him to do something, and when he's unable to do so, he'll get killed. As such, the Spy should have no problem with running back to base and using the refill locker. It not only restores health and ammo, but also instantly recharges the power to your cloaking device. Plus, it extends the length of your individual life, so you can get more impressive "most kills" and "longest life" statistics (if you're into that sort of thing).
-Understand the limitations of the backstab
If you're trying to backstab someone, make sure you're actually aiming at their back. With characters like the Heavy, it's easy to assume your attack will be an instakill backstab, considering roughly 76% of the character's body seems to be composed of his temptingly massive back. I've suffered many a death due to this faulty thinking.
You'll know you're actually going to backstab someone when your onscreen hand changes positions; the Spy will alter his hold on the knife so the blade faces downwards instead of upwards, in a very obvious "backstab ready" sort of image. It takes a bit for this animation to complete, but when it does (or at least, when it initiates), you'll know you've really got his back.
The animation becomes unnecessary if you're, say, chasing someone and they happen to be just far enough that the animation doesn't begin, but close enough so that, even with the knife's relatively short reach, the blade will still hit their back and thus cause an instakill.
-Choose your battles carefully
A Spy does not usually get a second chance. People become more wary after they see or experience a death at the hands of a Spy; players tend to look around a lot more, fire upon their teammates more frequently. As a Spy, you will only have the opportunity to catch your enemies completely be surprise for the first few minutes of a round; after that, they'll know a Spy is coming, in some form or another, and they will prepare. Don't try to kill every enemy you meet, as not all of them are worth the trouble, and it only takes a few kills for the entire team to get wise to your scheming.
-Wait at least thirty seconds after using a teleport
For thirty seconds following teleporter use, a cloud of bright red or blue particles (again, depending on your team) will follow you around, regardless of whether you are cloaked or disguised. If a Spy runs into a blue base disguised as an Engineer but has a cloud of bright, neon red teleporter particles surrounding his body, he might just as well have stopped, typed "I'M A SPY" into the public chat, and then gotten rid of his cover altogether.
Generally, I'd just suggest not using teleporters at all, but depending on the map it can sometimes be quicker to take one and wait the thirty seconds than to walk the entire way. Use your judgement.
-Don't assume that people will stop shooting at you
Just because you've stabbed someone and then activated your cloak doesn't mean that (A) people can't see you and (B) that they are firing blindly at you, and that their bullets are only hitting you due to pure chance and their unusually keen ability to predict your location. If you are directly shot by an enemy projectile, your body will glow that same red/blue color it does if you bump into someone; ergo, if you run away but are shot in the process, the enemy will know where you are, follow you, and, if you don't manage to change course so they miss and lose track of you, then you'll probably die.
I've also seen Spies so confident in their disguising ability that when someone fires at them, they don't run away. These Spies assume that if they act like the fact that they are being shot at is no big deal, then the enemy in question will immediately assume their authenticity and cease their fire. This is a bad idea, in that it incorrectly assumes that the enemy team is comprised entirely of idiots.
Yes, they may not have a reason to fire at you, but they could just be thorough. Or, perhaps they just saw a teammate with the same name as the one on your disguise, and since you aren't firing your gun, then they've assumed you're probably a Spy. Or -- worst case scenario -- you run into the very enemy whose name you have been assigned, in which case it won't take long for your enemy to realize that there cannot possibly be two of himself.
The Spy may die a lot. This is okay. His purpose is to quickly and decisively destroy the people and structures whom others cannot reach; if a red Spy makes it behind enemy lines, stabs but a single blue Heavy who has been pestering his red brothers, and is subsequently killed by the Heavy's Medic pal, the Spy might be disappointed -- but he shouldn't necessarily be.
Obviously, one would hope to have a kill/death ratio greater than 1:1 and the best Spies should be capable of this, but there's no shame in losing one of your lives in order to take out one of their forces. Especially if the enemy in question could have caused a significant amount of damage to your teammates.
So, yes, try to stay alive for as long as possible, and don't be reckless, but don't feel bad if you get through a life having only killed one enemy; so long as they were a sufficiently important person, you've done reasonably well for yourself.
-A revolver is an admirable tool
Your revolver, while inaccurate, does pack quite a punch. If you meet a character whom you cannot backstab -- like, say, a Sniper with his back to a wall -- consider pumping him full of lead. Given the fact that the knife is far too slow and weak to net you too many non-backstab frags, a revolver is always the best option for taking down a single enemy if you can't get to his spine. Get close, and empty your six-shooter into the prick.
When playing against him:
Pay attention to the kill notifications. If you see that someone on your team was backstabbed, be wary. If you're a Heavy, memorize the names of the Medics who follow you, so if you see their name on the upper-right corner of the screen with a little "backstab" insignia next to it, you'll know to immediately turn around and start firing.
Shoot your teammates. Frequently. Unless you're low on ammo, there's no harm in double-checking to see whether your teammates are Spies. If you happen to shoot a Spy who is disguised as a member of your team, you don't always necessarily have to fire at him enough to kill him: in certain cases, with less self-assured Spies, simply being intentionally shot at will sometimes be enough for them to lose confidence, drop their disguise, and either engage you in direct combat or run away. The Spy's main weapon is psychology -- turn that crap against him.
TURN AROUND. The Spy can't kill you if your back is to him (unless you're really low on health and he's got a revolver).
Take a meandering path. I stated this in the Soldier article, as it works best with him; still, any class can still benefit from erratically moving from side to side as they move so as to potentially run headfirst into any cloaked Spies moving in the opposite direction.
Pyros, baby; their flamethrowers are invaluable in the fight against Spies. As said in the earlier article, Pyros can't light their own teammates on fire; they can light disguised or cloaked Spies aflame. Even if they're invisible, the flames that slowly eat away at their health won't be.
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