This is a film where everything happens in the spaces between words, between lines, between scenes. It's a... what's holier than a Swiss cheese? A ciabatta of a film, but tasty nonetheless.
This is a film about the Wall Street collapse of 2008 that barely attempts to explain the insanely complex financial shenanigans that caused the crisis. It feels as though the filmmakers decided that the audience is never going to understand it anyway, so let's go ahead without explaining it at all. Though there is some explanation late in the film, and as one critic said they play the "explain this to me in words of one syllable" card a bit too often, the key here is that you don't need to understand the finances. All you need to understand is how important they are to the characters, and the top-notch cast makes that abundantly clear through a variety of understated techniques.
Another way in which this film takes place in the gaps between lines is that it depends a whole heck of a lot on the audience's understanding of the characters' world. If this film somehow fell through a time warp to the year 2000, no one would understand it. You need to have at least some understanding of the 2008 financial crisis to understand the plot. You need to know that when one character flips another character a small black object (which barely even appears on screen), and later that second character pulls the top off of something that looks like a lipstick, that it is a USB thumb drive... and what a thumb drive is, and how it is used, and what it can contain. When two characters are sitting at a bar, and you hear a buzz, and one of them glances down at his lap, and they both leave the bar without a word, you have to know what text messages look and sound like and what they can mean.
When I was in high school I took an acting class in which we memorized a very simple, meaningless dialogue1 and then had to present a brief scene using that simple script to express a relationship between two characters (first date, estranged lovers, father and son who's going off to college, etc.) -- it's all in the intonation, the body language, the pauses, the subtext. Practically this entire movie is like that. Much of the dialogue is banal, and the action restrained, yet the actors manage to convey the emotion and importance of the situation.
And the situation is important, dramatically important. There's a lot of tension in this movie, even though we know how the 2008 financial crisis ended up.
I commented to Kate on the way home that "this is a science fiction movie, and the science is economics." But, as she pointed out, that isn't really true; it's not SF because there's nothing in it that didn't actually happen. This is, nonetheless, a fabulous example of how you can take a plot that is made up of technobabble and mathematics and turn it into a story about people and emotions. I'd love to do something like this in SF, but as I mentioned above it depends so much on the audience's understanding of the history and technology that you would have a real tough time writing an SF or fantasy story that still worked if you left out as much as Margin Call leaves out.
So, in summary: not a fun movie, but one that's worth studying.
1 I still remember every word: "Hi." "Hello." "It's been a long time." "Yes it has." "How've you been?" "Do you have to ask?" "No, I suppose not." "Did you walk?" "No, I got a ride." "Oh."