The informal interface is made to almost maximise the noise in the communication, which lowers the meaning that can be extracted from the message. That is, it's intended to be so easy and fit in with all other activities that it can be done almost accidentally, unconsciously. You can't, when you're glanced at, assume the person is thinking really hard about you: they might be filling in time. This is a communication theory thing.

Oh, by fitting in with other processes...

Raskin argued for keys over a mouse for the desktop interface because keys were quicker. But the mouse won out, why? Because the mouse gives time to think. (This is Tog's argument, I think.) Yes, keys are quicker, but a mouse gives you time to think (because you're not looking for keys) and so you can anticipate the next step. It's that sort of time that lets you have awareness of what else is going on, the people around you.