It is more dynamic than in most games. Enemies "see" or hear you (no through-walls onslaught when you're hidden like some other games, Uncharted included).
A "balance of power" system is also used. Enemy behave differently depending on what weapons they have and what weapons they know you have (and not knowing if you have bullets for example, as demonstrated) and depending on how many of them there are.
A lone guy with a plank won't attack head-on if you have a gun (but might try to flank or surprise you) but a group with mixed weapons will if you only have a bow. An enemy is more likely to flee if you've successfully killed others in his group.
As for Ellie her AI is constantly evaluating the environment, sticking with you, looking (and listening) for enemy, locations to hide, items to use as weapons (guns included), tries to flank the enemy or hide so that if your (Joel) attack or defense doesn't pan out she can attack, or distract to open up a new opportunity.
There is always an element of weighting and scripting involved. Ellie and the other survivors, hunters and clickers aren't real people, but they behave more realistic than in most previous games.
The Last of Us is "wide linear" meaning that there is a linear story and generally linear level design (as opposed to sandbox open world) but you can navigate through with much more freedom than in the Uncharted series. There are multiple paths through scenes and you have the choice, as in the gameplay demo shown, to go in shooting, draw the enemy to you and take them out, or sneak past and avoid conflict. The limited ammo and other supplied, persistent health, and real-time crafting system create an intensity that often drives your decisions.
Hey Dan and Laura, seeing more coverage of The Last of Us is good but you need to do some basic research other than just watch the demos at E3 as all of what you are unsure about has been known for weeks (even before E3).
It was a pandemic, the Cordyceps fungus, that wiped people out. The gameplay demo is set in Pittsburgh (same location as the two previous trailers), in fact it is in the *same hotel* as the original trailer (ok, that's never been revealed before now). Joel agreed to take Ellie out of the quarrantine zone and across America on a promise to a friend who died.
The survivors you encounter in the demo are "hunters", known to kill "tourists" like Joel and Ellie without a second thought, just to get more supplies. Joel and Ellie have to get past them to continue on their way to the bridge to get out of town. That sets the scene for the killing spree shown in the demo but as you said you can use stealth; you can sneak through without a single shot being fired.
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