I was fortunate enough to watch from the front row. One of the delights of seeing anything - whether it's a theatrical performance, a rock show or a sporting event - at such close quarters is that you can see the performers dealing with the tiny practicalities of their trade. You see the looks exchanged between them. You can tell that chair has been moved because something is about to happen in the place where it stood. You can see somebody being handed a prop that is about to play some part in the action. You notice when somebody covers for somebody else. At certain angles you can see performers in the wings getting ready to come on. When Sahr Ngaujah came downstage drops of his sweat fell into the front row.
During previews the cast are getting ready to face first the press and then the general public. They've done their technical rehearsals and their dress rehearsals. The previews are about ironing things out and getting up to speed. The only thing which appeared to go slightly awry on this occasion was a piece of the set that refused to move. The actors and the lighting crew were so quick to adapt it's unlikely anyone beyond the first few rows even noticed. When the show had moved on I could see from my angle a technician attacking it with a screwdriver. Apart from that you would never have believed they hadn't been doing it for months. It was their first preview. The level of accomplishment beggars belief.
I love doing anything with actors because they always assume things are going to go wrong. They rehearse and rehearse even the tiniest things, not so much to make sure they don't as to ensure they are mentally ready when they do. I'm sure that's a lesson that applies far beyond the theatre.