When my workshop was in session , the members of the workshop at TheatrGROUP would go as a group to see a play being performed somewhere in St. Louis (typically on a Friday or Saturday evening, so we could discuss what we saw on Sunday, in the workshop.

We saw a play one evening in which part of the action in one of the scenes involved a group of right wing radicals who had staged a robbery.  One of the actors was stationary at the vault, and as another would run past him, he would throw a bag of "money" to the actor, who would catch it in mid-stride, and keep running.  When the last actor to receive the money ran past the pitcher, he failed to catch the bag of money.  What did he do?  He kept running, without picking up the bag of money.

The actor who was tossing the money then ran offstage in the same direction as the others.  Did he pick up the bag of money?  No.  He ran right past it like it didn't exist. He had plenty of time to grab the bag, but didn't.  Why?

Because it wasn't in the "blocking", I guess.  The actor couldn't do it, if it wasn't "blocked", if it wasn't "the way it was rehearsed", or "the way it was supposed to happen."

What did the audience think when they saw that mistake?  Probably something like, "Oh, that poor actor missed that bag...I'll bet he was embarrassed." 

What did the members of the workshop think?  "Why didn't the actor who was running just stop and pick up the bag?" I replied that he was probably afraid to break up the "tempo" of the scene, which was all action and no substance, so he just left it there.

Another student asked, "But why didn't the actor who was throwing the bags of money pick the bag up when he ran offstage? "  I responded, "For the same reason.  It wasn't rehearsed that way.  For them, the only option is to ignore the mistake as though it didn't happen, rather than destroy the director's "pacing".

Maybe you've seen actors accidentally knock a drink off the table, and leave it where it fell, "pretending" it didn't happen.

Living "moment-to-moment" on the stage allows the actor to accommodate any unexpected changes in what has previously happened in the scene.  Just as in life, you never know when your best friend might, in a sudden outburst of anger, smack you in the face.  But you deal with it anyway, and get on with the scene.

The actor chooses a sequence of objects on which to concentrate in a play.   He then applies his full concentration to those objects.  But he must be prepared to deal with events and circumstances which cause his object of attention to momentarily change.  And he must deal with these changes in the moment they occur, and with the same logic life itself would dictate.

Once in a while, something unexpected happens on the stage that ends up becoming one of the more memorable moments of the play.  When the actor is confident enough to live "moment-to-moment" on the stage, he finds himself praying for these "happy accidents".

At TheatrGROUP, I will only block scenes when absolutely necessary for purposes of lighting, or to emphasize a specific moment in a scene.  I have found that when the actors here are left to their own resources in rehearsal, they usually justify every movement on the stage much more realistically than I could do so by concocting a blocking diagram.

And where they are onstage can change nightly, as long as it is justified by the actor.

Yes, I am a lighting director's worst nightmare.