I've seen it thrown around a lot on the internet: The stealth is broken. The AI is dumb.
I'm here to tell you how wrong those people are, and help those frustrated with a seemingly unpredictable AI. The game is harsh in it's realism, it doesn't tell you anything, the cues of what the AI is thinking are few. By todays standards in shooters, Far Cry 2's AI is damned smart, and it does abide by the rules set by the game.
The second to last section I'll share some tips and discoveries that will aid you.

In recent years the community has discovered the infamous bouncing NPC bug was caused by high frame rates. The engine is basically over-revving, and things fall apart. The AI is also affected by this. It wasn't a problem in the first couple of years of release, as it was quite a demanding game.
Similarly, I ran into issues all those years ago when I used software video capturing for the game. It ate up a lot of CPU, and thus I found the AI being basically blind and deaf in many cases.

To play the game with the AI acting as they were meant to, you need to limit the FPS to <60. This can be done by setting the video resolution to 60hz and turning on vsync. Or using an FPS limiter.

Most of this guide is based off experiences playing on Hardcore difficulty setting. Each step up in difficulty adds to AI awareness and reaction times.
But when you hit infamous difficulty, a lot of flags get toggled, evident in the game files, and you get little mercy when sneaking. You also do not go into critical health where you can perform the grizzly first aid actions, you just keel over. It has heavy impact on gameplay, it doesn't simply touch your health and ammo as you might expect.

Consider what you want from the game when you choose. Infamous gives you little room for learning and experimentation.

This is of course the most important. To be safe, have patience and use their blindspots, move from cover to cover when they're not looking in your direction.

Sound attracts attention. Even when you think you're far from any enemies, discharging your unsilenced weapon, or playing with explosives, is obviously heard from far far away. You risk putting enemies you've not even seen yet into the alert phase, and then they're already looking for you and ready to shoot on the slightest sighting of you.
Move slowly, the faster you move, the more noise you generate. Sprinting is loud. Using keyboard&mouse, you're the most quiet when crouched and aiming down sight. This is necessary when you're moving up behind them and stalking.
Be wary of bumping into physics-enabled objects, I have a suspicion this has brought attention to me before.

When you can't break line of sight, often the case, that's when the enemy's detection range, and your visibility radius, comes in to play.

I see the system as circles and cones. Your visibility is like a circle around you, the radius increases depending on various factors listed below. The enemy's field of vision is a cone separated as outer and inner. If your circle intersects with his outer field of vision he will get suspicious, and if it intersects with his inner field of vision, he'll spot you and go into combat. (if this doesn't make any sense outside my head, I'm sorry)


Their detection range is affected by:
Light - During day, their detection range is at their highest. Nights may not seem very dark, but it impacts their detection range greatly. More than 50% less than during the day I'd say. Moonless nights are the darkest.
Weather - Weather also impacts their detection range. Windy weather and storms lowers their range due to all the motion in the vegetation and tress, masking your movement (and possibly your sound). Clear skies aid their vision.
Hell Diguner suggests that during high noon and simmering hot weather their range takes a slight hit. Experiment away.
Special cases - Snipers and RPG nests have a much longer detection range.

The best condition for sneaking is during a stormful night.

Your visibility radius is affected by:
Movement speed - The slower you move, the smaller your visibility radius. Everything that moves in a scene stands out!
Stance - Your default state when sneaking should of course be the crouch, as it's a substantial decrease of your visibility radius, as well as lowering your movement speed (thus sound).
Blending in - Bushes are your best friend in the jungle (the ones whose branches will move when you walk into them). It breaks line of sight like a solid object. Moving into one allows you to observe your surroundings safely, as long as you're careful not to walk too far into the bush so part of you pops out on the other side where the danger is. Once bullets start flying, bushes won't hide you if the enemy sees you crawl into one.
Tall grass is your second best friend. Stay in it at all times during sneaking, it makes you a lot less visible than standing in the open. (Buy the camo suit to decrease your visibility range in tall grass even further)
Water - Diving into water greatly lowers your visibility. The deeper you swim the better. (Thanks to Hell Diguner for reminding me)



You get a sense for their detection ranges and your visibility radius after some time. The more you experiment with sneaking, the faster you'll develop it. HINT: Quicksave before starting to sneak to avoid frustrating failure after frustrating failure. Once you're starting to see success, it's time to take the failures like a man.

Their chatter is the only pointer you have of their awareness. Listen for it not only to know their state, but also their location in relation to you.
Relaxed - Coughing, grunts, sighs, humming, casual chatter; they probably don't know you're there.Their primary weapon will usually be on their backs.
Suspicious - Did their idle chatter suddenly become interrupted? You'll want to relocate fast, because they've heard or caught a glimpse of something, probably you. You'll hear clattering of their gun(s) as they pull it off their back, and they'll investigate the area of suspicion, sometimes letting the rest of their group know. They're not yet alerted.
If you stay out of sight, they'll go back to relaxed state after a while.
Alerted - They know you're there, but not where. Usually this is caused by a violent sound (engine noise counts), or the sighting of a dead body. They'll spread out and search for you with their trigger finger at the ready. You'll know you've been spotted when bullets starts flying.
Combat - Obvious. They know where you are, and are unloading everything they got in your direction. Staying out of sight and keeping your guns cold for a while will return them to the alerted phase, giving you the element of surprise once more as they investigate your last known location.

Fun things to listen for: terrified speeches, pep-talking, taunting, frustration, them thinking there's more than one attacking them.

Wounded - If you wound an enemy, he'll drag himself to cover if possible, lay up against it, and pull out his pistol in a last stand.
Other team mates will react to their wounded, try to secure the area he's in, once deemed secure, they'll carry the wounded to cover. It gives you an opportunity to take them both out easily.
Fire - Fire spreads panic, getting away from fire takes priority over returning fire, making them easy targets. Causing a fire by shooting gas tanks from afar is a good way to soften up an area.

These guys are aggressive. They will not stop chasing you unless their vehicle cannot continue. Out-running them almost never succeeds.
Patrols only roam on roads that lead to a nearby checkpoint. If you're far from enemy checkpoints, chances of patrols are almost non-existent.
The moment you know you're being chased by an armed vehicle, get out of your vehicle (but don't get run over!), or if available switch to your turret, and prepare to return fire. A molotov to the hood of their car forces them to exit their vehicle.
Explosives are of course the fastest way of dealing with them. A well-timed handgrenade if you don't want to spend a precious RPG or M79 grenade.
I stick to armed vehicles when I know I'll be using the roads, pressing the crouch key (default: C), switches between the driver seat and gunner seat. This is my first reaction to an enemy patrol. Fight fire with fire. They're very easy to hide from on foot.
Hell Diguner adds that if you lure a patrol far enough from their patrol route, a new patrol vehicle will spawn and take its place. I'm guessing it's the same problem as the respawning checkpoints.

These are their guys on outlook. They have scopes and will slowly survey the area, and therefore have an insane visibility range. If you plan to avoid them, you need to be able to break line of sight whenever they survey across your location. Watch them closely, preferably through a dart rifle scope should things turn sour. Hell Diguner points out that being spotted by them doesn't necessarily alert the nearby enemies. This is also my experience.
They're usually safe to take out silently as they're isolated from the rest of their group. The dart rifle was made to pick them off.

The biggest flaw of the game is no doubt the checkpoint system. There's no timer deciding when they repopulate. Once you get a certain distance away from an area, that area unloads from your computer. The game doesn't keep any track of cleared checkpoints or the like, so when you return and the area reloads, it'll be in it's default, populated, state. Keep this in mind.
If you really hate finding a checkpoint you just cleared repopulated, don't plan your return route to go through it. Sounds dumb, but even a hardcore fanboy like me is taken a bit out of the experience when I have to clear the same checkpoint twice within 5 minutes.

This is about how far I could go before the checkpoint respawned. About a third of a map at default zoom. But needs to be tested more, so don't count on it yet.

First and foremost, plan ahead, experiment with the tools. Be creative. Be smart. Put yourself in the situation, what would you do? Rarely do what other shooters have taught you. (such as making bee-lines for objectives and attacking head-on) Remember, the journey is part of the mission. As soon as you accept a mission, the mission has begun. Every checkpoint and patrol is a secondary objective that must be bypassed to reach your main objective.

I think half the fun of Far Cry 2 is experimenting to discover more ways to engage, so consider some these potential spoilers. (I guess)


Stealth takedowns - To silently take someone out, with a suppressed weapon of course, always aim for the head (even with the suppressed shotgun). Clean headshots are the only "silent" kills. Any other hit or miss near him and they cry out, and then it's straight to the alert phase. If the second shot is landed fast enough you can get lucky and not raise attention to others around.
If there's people very close by the guy you're taking down, chances are also they'll react. I suspect it's due to the sound of the body dropping to the ground. Go for isolated targets.
The machete I never use the machete for takedowns, it's both hard to get close enough without them turning around, and the backstab-slash is not a one-hit kill. You'll have to stab him while he's down. As you can probably guess, the enemy isn't going to be quiet during this. Succeeding silently seems more like a stroke of luck than a skill. Though there is that feeling of being a wicked, sadistic mercenary.

Recon - Before moving in, count the number of enemies. Gauge each target's threat; a shotgun is not going to give you much trouble at a distance compared to a rifleman. A sniper or RPG will screw you over fast. Take note of opportunities, stashes, mounted MGs, barrels, propane, cover.
Watching the consequences of your planning is usually much more fun and rewarding than charging in blind, guns blazing.

Distracting the enemy - Explosions are a great way to cause a distraction. It'll bait them out to investigate. And if you're smart, you aren't and weren't near the explosion.

Gliding on the water - At night, it is often possible to glide past enemies on the river bank on a swamp boat. Before getting too close to them, get maximum speed, and exit the drivers seat. It'll kill the engine noise, and you'll continue gliding in the direction you're pointed at for a while. If you're lucky, far enough to start up the engine again and continue without raising attention.
Fyre The Pyro adds that with the camo suit, you can do it in daylight as well. They don't realize till their chase opportunity is gone.

Shooting through cover - Is actually possible if the material is thin enough. This goes even for you. Find solid cover.
Once, in a moment of complete badassery, I silently took down a guard outside the shack I was in through the sheet metal walls.

SLIDE - I spent the first many many hours not knowing you could slide in the game. Sprint, and while sprinting, press crouch, you'll execute a slide on your knees. It's very useful for sliding into and between cover, and just feels awesome.

Weapon condition - The game tells you that weapons in poor condition will be more likely to jam, this is true. What it doesn't tell you is that the worse condition a weapon is in, the worse the accuracy, and higher recoil. So swap out your weapons that rely on their accuracy often with new ones from your personal supply.

Weapon crates - These seem like a waste of diamonds at first, but you quickly learn to appreciate them. Any weapon you store in them will be available in any safehouse you've unlocked. This means you can plan ahead, have two loadouts. When used right, they make you a lot more versatile.
Example: I'm to take out a cargo truck, its route goes through huge open fields. I decide to use the AS50 anti-materiel sniper rifle. The shortest route takes me through multiple checkpoints, and the majority in closed environments. I put the AS50 in the crate, and equip a shotgun.
Once I've made it to the nearest safehouse by the truck's route, I pick up the AS50 from its crate. The right tools for the right jobs.

Buddy-rescue - Keep your buddies rescue-ready, they're your safety net, your second chance to keep the ball in the air instead of abruptly ending your story. Visit a safehouse, take a nap, and you'll usually wake up with them next to you, ready to assist once more. Creepy.
Note from Hell Diguner: Sleeping will respawn patrols and guard posts, but not buddies. Be sure your buddy de-spawns by moving far enough away from them, or you may accidentally send them to an early grave.

Xbox360 controller - Driving with one of these is a lot more fun. More importantly, with one of these, you have total control over your movement speed, creep as slow as you want; it does affect your detection radius and noise level. But it sucks for aiming. So put it in your right hand and use it for movement, the mouse will still function unhindered. Good for sneaking, bad for combat due to lack of buttons when used this way. Put it down when fighting.
If you don't have a Xbox360 controller or other xinput controller, you can "disguise" any DirectInput (pre-Xbox360 era controllers) to be a xinput controller with the help of x360ce[code.google.com].
Personally I use a (DirectInput)Logitech G13[gaming.logitech.com], best of both worlds.
I've made a workaround to get the full benefits of analog movement while using mouse and keyboard.LINK

What do you think? Did you learn anything from my guide? Did I make grammar mistakes? Got anything to add? Let me know in the comments. And please rate up if you found it useful.