A good screenplay or novel needs some glue, by which I mean something that connects and holds together the people whose fates you wish to connect. The less possible it is for them to escape, the more conflicts you can pile on and the more plausible the emotional intensity of their predicament.
The two most-used kinds of glue are family and work. It's not impossible to get away from your family but it's certainly not always easy. The same goes for work--sure, you can quit a job but if you have nothing else lined up or likely and no money saved, that's not easy, either.
There can also be emotional glue between two people in conflict--co-dependency, for instance. Probably the best dramatic example is "Who's Afraid of Virginia Woolf?" in which George and Martha's love-hate relationship gives them both something to live for.
Recently I saw "Carnage," and while I thought the actors were good (especially Christoph Waltz) and there were some good amusing black comedy moments, it just didn't make sense that the other visiting couple would stay there once things got unpleasant.
It would have been relatively easy to create a device to keep them there. For instance, the power goes out and there's no way back down to ground level because the elevators aren't working. Instead, the lure was "oh, come back and have a cup of tea."
It's possible that we are to think that neither couple is willing to leave until they have convinced the other of the correctness of their position, but that certainly doesn't ring true for the Waltz character, who is in the middle of covering up a potential crisis for the pharmaceutical company he represents and is rather bored by this charade of handling things in a civilized manner.
The film is based on a play ("Gods of Carnage") by Yasmina Reza, which won a Tony. Reza and director Roman Polanski worked on the screenplay together. She also wrote "Art," another story designed to expose the pretentions, hypocrisy and smugness of the upper middle class. With both of these it's hard not to suspect that they are designed to make the audience feel superior without much danger of turning its view inward.
Leaving all that aside, from the perspective of crafstmanship I think the main lesson of the film is not to forget the glue.
(For more tips on plotting and characterization, check out "Your Writing Coach," published by Nicholas Brealey and available now from Amazon and other booksellers.)
