This game isn't necessarily split into parts... at least not officially but there are certain segments that make the game feel as though there are 3 acts of sorts. Today I will describe act 2, which unfortunately is extremely similar to act 1.
So at the end of act one we find ourselves in a tomb whose history is explained very briefly at the beginning of the story which in itself isn't really an act but just some sort of reference to how this now underground temple / fortress came to be. Our team of mostly whiney researchers and marines are all trapped at various portions of the cave temple, which makes very little sense seeing as how they all fell into the ground in close proximity to one another when the earth opened up while they were on the surface. You just have to suspend your disbelief and move on with this part of the story.
You travel from room to room looking at various items that explain the research team that had been here before and find all of their notes and recordings from 60 years ago or something and this brings about a really major plot hole in the story: It seems to me that if a well-funded group of researches ventured off into an archeological site that they might have told someone back home that didn't come with them that they were going to do so or at the minimum, some locals might have at least been involved as porters yet no one knows that this place is here until the ground opens up and swallows our very small team of adventurers.
We have significantly more interaction with the baddies in act 2 than we were in act 1. In the first part of our story you mostly just see them scurry by in the distance or the shadows but in act 2 they seem to be a bit more pissed off about your presence and now have a greater desire to kill you.
This is where things start to get a bit boring since every segment of the "action" is just a repetition of the scene before. You hear them coming, you shut and lock a door, they break through that door, and you retreat to another area where the creatures for some unexplained reason lose interest in pursuing our team long enough for them to have long drawn-out conversations with one another.
To me this is really boring. I am not really a "story-driven" fan when it comes to gaming and that is really all that this game is. Aside from quicktime events, you really don't control any of the action. There is not a single portion of this game that I have encountered where running away involves any sort of controller dexterity: The person you are currently controlling was going to go the way that he goes regardless of what button you push. There are large swaths of time where you aren't controlling anything at all and where you decide to move has near zero impact on how the story progresses.
Like I said, I didn't pursue every option available because it isn't clear what those options are, if there even are any.
Act 2 continues with a number of dialogue options but as far as I can tell what you decide to say only has an effect on your relationship status with other members of the team. I can not determine if this alignment has any impact on how the game progresses. I'd like to believe that it does, but this game is so long that I am definitely not going to do another playthrough just to find out if being a dick with every statement that you make somehow results in the game being better or even more difficult.
There is clearly an objective to keep as many members of your party alive as possible but even the times where I tried to die, I couldn't. I suppose if you repeatedly and intentionally hit the wrong buttons that the baddies would catch up with you and you would get sent back to a checkpoint of some sort but even then, I can't imagine that this has much impact on the overall story.
From what I can tell thus far is that your choices have minimal impact on where anyone goes and while I am not 100% sure about this, I think that since the cinematic scenes already take up a ton of space on the hard drive, that no matter what choice you make you are going to end up in the same place that has been pre-determined anyway.
I wouldn't really say that I am enjoying this game but since I have come this far I may as well go ahead and finish it. In act 2, which I finished yesterday, I was kind of hoping that the game would be over soon because honestly, it is quite boring and feels more like a Choose your own Adventure book from the days of when people read books.