What stealth elements are there in Tomb Raider?
I'll try to answer your questions based on my experience with the game.
- Walking/running and different types of floor don't matter. Not sure about water though, but I don't think it would matter. Changing arrow from normal to flaming also doesn't matter (not sure about if they can see, but a guard didn't hear it yesterday when I accidentally switched arrows behind his back). I also managed to knock some objects quit near enemies and they didn't notice. However, gunfire matters, they will hear you and immediately know where you are. Also, hanging salvage boxes that you can ignite draw attention when they drop on the ground (this happened to me yesterday).
- As for lightning and sneaking, the game seems to be pretty forgiving about those. As long as you don't run directly in front of enemies, they don't appear to notice you. Stay in cover if they walk your way. Sneaking on people is easy for as long as they look different direction (it's even possible partially from the side, although that's risky), but the sound of your movement doesn't attract attention.
- if you are revealed, I didn't find a way how to get back into stealth mode. They are pretty much after you and won't stop until they see your corpse.
- enemies alert each other when they spot you or when you injure them; they start yelling for help. One of the game tips says that you can see in survival instincts view which enemy will get help (should be marked red), but I never saw it working.
- if enemies find a body, they will move in to investigate and then stay alert and search the immediate area, but they will remain oblivious to you. No idea what happens then sorry, maybe they'll go about their business shortly (can anyone confirm that?).
- I believe there are areas where it is not possible to sneak around, they will detect you as soon as you enter which triggers gunfight. You will know it because enemies will shout "That's her, kill her!" In other areas you can sneak in, eavesdrop on conversations and try to remove the opposition silently.
- I only played on Normal difficulty so far, cannot say if difficulty changes something
Additional tips:
- Bow is silent, I use this for stealth kills. One headshot kills most enemies (you get a diamond-shaped headshot-crosshair), but there are enemies with armour that cannot be one-head-shot or at least I couldn't do it (even with fully charged headshot). They can probably be killed with melee sneak attack, I didn't try it yet.
- Silenced weapons are also quiet enough for when the guards are not too close to your position
- You must one-shot the enemies silently otherwise they start yelling for help. If you can sneak behind an enemy, use melee attack to kill them silently.
As for the Summit Forest, you can kill all guards silently there, just make sure to wait until the initial conversation ends. After a few first guards are taken care of, you can assume position in the trees, observe their patrol paths and kill them from up there. Good hunting.
In addition to the first answer:
- In certain parts of the game lighting your torch with alert the enemies. e.g. at:
... the Chasm Stronghold level, when there are dozens of enemies around and you have to remain undetected. Lighting your torch is a sure way to get killed.
Comments in the first answer mention that using your 'Survival Instinct' (Q by the default in the PC) will highlight enemies in yellow or red, where yellow won't alert anyone else when that enemy is killed while red will alert other nearby enemies. In my experience, this isn't true in all cases. I have successfully killed enemies highlighted in red in Survival Instinct mode without alerting anyone. Just be sure to check if the enemy you want to kill has no one facing him.
In the Summit Forest, I have missed an enemy with my pistol with silencer attached and hit a nearby surface instead, but no one was alerted. It seems that even the projectile effects themselves are silenced if you have the silencer attached.
Also, in the Summit Forest: enemies will utter something like "in the water!", if they hear you splashing around in the water, but I haven't noticed any behavior changes. They weren't alerted and I wasn't spotted.