Perfect answer, thank you!

Plays tend to be a bit more direct in their dialog, because you really have to get everything across the first time around. Plays aren't seen over and over, the way movies are, so the writing is going to have to be a bit different. Also, there aren't any close-ups or jump cuts or anything else to tell the audience the story, so it all comes down to acting and writing with the director doing what s/he can to keep the staging interesting.
Others might have more to say about this, but I know in the theater actors have to "play to the back row," which means that everything is generally a little bit bigger. For this film, however, I thought that it might just be the director's choice to make it all really stylized to give a sense that the characters are just "play-acting" at normal life to avoid more disturbing matters. I've never seen the play, but it seems like a bit of a jab at WASP culture, as well.

I can't say it has a name, but it is very similar to theatrical acting, and writing. Plays you might see in a theater will have similar writing and acting.
I've seen things that would make you want to write a book on how to puke.

I bet it is easier than regular acting. It seems so amateurish, I bet I could pull off being in a scene from this movie.