Aristotle teaches us that action is the basis of all theatre. Whether this be the plot, the characters, or thought, a play is completely useless without action. Strangely, both An Interlude and Shift were, technically, very low on the action. In An Interlude, both the man and woman, according to the script, never move. They simply sit facing each other, with the woman holding a notepad. Essentially, this shouldn't be good theatre...but it is. Because an action can be verbal too: speak, refute, ask, argue, interrupt, etc. While physically this play might very well be boring, it has a lot of competing actions through the thought processes and words of both characters. With just this, they set the scene, display their personalities, explain the conflict - multiple conflicts, actually - and a sort of trailing-off resolution.
Meanwhile, Shift has a good deal more action. In fact, the script doesn't function well without a few stage directions. While the words are important, the actions of the characters show much better what their situation is, how they're dealing with it, how they relate to each other, etc. The only thing physically seeing these characters on stage wouldn't show us is the circumstances of the city: the Shifting. That's a strange enough idea that it must be explained, though it's done casually here. However, everything else is action, covered by simple, pretty words. You may sense it through the script, but the tension between the characters - the real conflict - must be shown.
So it seems Aristotle is right...though I'd like to point out that, since life is a series of actions, and plays are always about life in some way, the conclusion that action makes up the theatrical story is sort of, well...obvious.