Warning: this musing will contain minor spoilers for Sanctum. You have been warned.
Sanctum, in my mind, presents a perfect case study on the first act of the movie, and how one really needs to be careful with characterizations in the first act, seeing as how they can lead to many problems later. It also presents a study of which characters you should not focus on if you're going to go with a survival tale with a very small amount of people.
My first gripe that Sanctum religiously follows: it spends too much time on characters that don't have any actual bearing on the story. In the first act of the movie, we're introduced to three characters that are in one area of the cave. They're there when the movie starts, kind of doing stuff. And what happens to them? One of them drowns to death before the first act is even given a chance to finish and two of them are never seen again after the shit really hits the fan for our five main characters. Granted, one of the characters was needed to show the dangers of the cave, but the other two? They're not mentioned at all. It's probably justifiable given the plot, but it still feels like they're cheating by attempting to develop two characters who don't ultimately have any bearing on the story in any way, shape, or form.
My second gripe? It inserts too many pointless sequences. Those helicopter shots with the majestic music playing? Uh... is it too late to mention that we're not even going to be seeing that majestic eye-candy landscape you just showed us for half the movie? If you're not going to see it for half the movie, then why focus on it? Helicopter scenes don't need to always have sweeping views, you know; you could develop your characters a little further.
On that note, there are also a lot of pointless shots. There are two or three shots of this native guy who simply looks up at the sky. I don't know what the hell those shots served, but they ultimately didn't do much for anything, really. And the scene with the young hero drowning in water at the end? In this case, it's also pointless, given the ending of the movie. And if it was meant to be in his head or whatever, you probably should have clarified that in the way the ending was edited, because it doesn't help when it looks like one of the characters is going to die, but *gasp* he doesn't because he's the last of the main characters who's still alive by that point.
Speaking of characters... the third gripe I have is that it doesn't know which character assets it should focus on. At one point, one of the characters talks about how he has daddy issues and how he doesn't like his father, who just so happens to be in the cave. Okay, movie, that's good and all, but don't give us the same exact conversation twice! Your viewers are not morons. Once is enough for us to get that he has daddy issues. And then there's one of the other characters, who apparently had an accident before the movie's events. Apparently, the movie didn't see it fit to explain this little ailment with the character until after the beginning of the second act. It would've been nice to explain what the hell happened to him before taking us on this great adventure, movie.
The fourth gripe I had with it was that it didn't do a good job of location-building in any sense. We get a CG graphic of the layout of the cave pretty soon after the movie starts. The only problem with this is that it becomes pretty pointless after a while since it only shows the mapped areas of the cave, and the action leaves the mapped areas literally five minutes after they get in major trouble. As well, it doesn't do a good job of setting up potential cave geography or signs to look for in finding a path outside of the water; therefore, some pieces of the pathway feel very contrived as a result.
This can be a little jarring when there are other things it does reasonably well. There's an incredible example of "show, don't tell" towards the beginning where one of the more eccentric characters skydives into the cave (no, he doesn't hit his head because of the mouth of the cave is huge) that helps hammer in his eccentric attitude for all of us. And then they manage to successfully set up a Chekov's gun with a shark tooth necklace that one of the characters wears that ends up helping to save his life. So when those two pieces of gold wind up in an otherwise uninteresting first act, the flaws only become more noticeable.
Overall, Sanctum has multiple problems in its first act, and it drags down the already mostly tepid second and mildly exciting third act down in quality as a result. It can also lead to some plot elements seeming to come from nowhere (it might've been good to know that bat shit is a good way to find a way up in a movie like this before the plot element actually came into play), it can lead to some maddeningly idiotic characterization moments (what the bloody hell was up with the father and son making up after a line that would've gotten any other parent smacked in the face), and it can leave us less invested in the adventure when we're stuck thinking what happened to the other characters. And the first act is one of the biggest reasons why Sanctum failed.
This is Herr Wozzeck Muses. I'll see you guys next time.