One important stage in the rehearsal process is when the actors rehearse their blocking... but the set isn't there yet!
Instead, the stage manager "draws" the outline of the future set on the stage floor using spike tape. (This is a kind of narrow gaf tape, or a thin duct tape, but in rainbow colors.) Sometimes the set designer has to help, if the set's floor plan is tricky.
Last night was a Designer Run for one of my shows - a full performance of the play, but last night at least, concentrating more on getting the new blocking down than on emoting. All acted out on a taped out set. This is a very useful run for the lighting designer especially, because he needs to know where everyone will be in order to point lights at them. An exercise in imagination!
(For the set designer the biggest amusement is usually to watch the actors walk through walls.)
This morning we start building the set. Gotta run...