how do you do real time voice acting dialog during gameplay in T3D?

#1 At a basic level, it is just audio triggers tied to actions rather than being tied to the environment. When actions are performed, choose the appropriate trigger (that's the tougher of the piece since it cannot just be a random choice). It is much easier to imagine in a tightly linear level where you control the player's (and in the case of tLoU, two characters') path through the level. And by prototyping the system in a tightly constrained level, you can find the commonalities between how audio choices are made depending on the environment and actions so that you can extend it into a more open level.

Plus it would take an incredibly large amount of audio and studio time to get a really strong commentary feel. Most of the narrative in that clip comes in the form of commentary "here's your ladder", "I'm on it", "alright now", that is situational. Through playtesting and usability testing outside of the team, you would need to find the balance where they "always say the same thing" or the comments do not fit the context well because there is too much for it to try to match situationally.

#2 Thanks David,
I'm not to worried much about studio time as much as finding voice actors/actresses. It nice to know that triggers tied to actions is the way to go. I'm sure the audio files for it would be pretty large, but I guess that alot of games nowadays are.

#3 If you want another example as to how it's done, you can look at games like Team Fortress 2, or any other game with 'battle chatter'.

When you do an action, such as reloading, (or with a few engine mods, which exist as resources, play an= given animation) you'd see if you need to play a sound.

Lots of the random chatter, where characters comment on the environment can be done via a level trigger, and non-situational but random chatter can be done after a certain amount of time has passed.
You'd just have a timer, and check if the current circumstances warrant a bit of chatter(wouldn't want to talk about how bored you are during a firefight!) and then play a given sound, or set of sounds. That's how bioware does a lot of their party chatter.

The kind of chatter being done also fairly heavily depends on the tone/type of game.
In a shooter, battle chatter would revolve around the soldier's status, enemy locations, etc.
In a game like Last of Us, most of that chatter is just characters throwing around commentary about what they see or are doing, so the requirements are a bit different based on tone. Stuff like battle chatter of 'reloading!' can be said a lot more often without it being too repetitive than someone commenting how hungry they are.

As for the audio file stuffs, yeah, if you plan on having lots of chatter or character commentary, they'll likely take up a large chunk of your game's size. Most modern shooter games have several thousand lines of JUST battle chatter, let alone story/scripted stuff, and one of the big reasons Kojima 'needed' blueray for MGS4 was because they were trying to compress the audio as little as possible, and they just squeaked in the 50gb dual-layer size.

#4 Yes I'll put more though in to it. I do want the characters to have some chatter but not so much that it gets annoying, but still believable.