I'm not familiar with the play God of Carnage, nor will I ever be, nor do I think the actors really care about it that much... even if they give interviewers the impression that they do. Carnage, the movie, is two things: a post-script to Polanski's early career fascination with the psychological terrors that can exist within the walls of an apartment (Repulsion, Rosemary's Baby, The Tenant), and an on-screen forum for a group of fairly talented actors to play a game of four square. Christolph Waltz snorts. Jodie Foster counters with a debilitating weep. Winslet comes stage left with a drunken hobble and a "faggot" blast. Then John C. Reilly stuns them all by getting racial with it.
Carnage is fun, and funny, and charging, but is it anything other than a long-form trending You Tube clip of the week with a little momentum behind it? Nah. Everyone involved has been a part of something much greater and more memorable (although, Jodie Foster's performance alone does stand out to me as one of her best). There's not even much wit to the material's eventual insight: that the brats on the playground and the rats in the gutter can take care of themselves just fine, it's the sophisticated upper-East Coast adults who are in trouble.
But, shoot, everyone needs to shallowly get their rocks off now and then, right?