This is the thing I always forget:
Rehearsing a show is fun.
Because you get to fiddle with all the little stuff. In between presentations (of whatever sort—readings, workshops, etc.) you’re spending your time trying to fix things that are genuine problems, like “The opening number feels disjointed and too long” or “We have three ballads in a row in the second half of act two, so we need to cut two of them or move them to elsewhere in the show.”
But once you get into rehearsal (assuming you’ve fixed most of the genuine problems you’ve been worried about) you get to play with the fun stuff. Like, today we decided that (for now at least) two characters who used to not sleep together are going to sleep together. We’re not sure it will work—there are later scenes that might be affected adversely by such a development—but we’re hoping we can keep it, because, hey, more sex is always better.