I remember someone telling me years ago that shows like to reserve some premium seats until the day of the performance in case a 'VIP' decided to come see the show at the last minute. Is that a real thing or do shows just sell all their seats as quickly as possible?
Yes, those are called house seats and they are purchased through the general management firm directly. Usually priced at the top non-premium price, without fees. People working in the industry and working on the production have access to house seats. Some people like authors or theater owners or stars have an allocation of house seats each night that they could buy for each performance (which becomes a major thing on a show like Hamilton or Merrily or Bette Dolly).
Unsold house seats get released as you get closer to the date (72 and 48 hours before), with the last pairs being released the morning of the show so they can be sold to the general public.
quizking101 said: "Usually VIPs require an extra layer of planning/security to discreetly get them in/out of the theatre"
That’s only true for a tiny tiny number of people. Like a sitting president or maybe Beyoncé or Taylor Swift. An age of masks has also made this a little easier (people like Leo DiCaprio wear a mask everywhere for attempted anonymity now).