You might want to read older memoirs of performers from the 60's and earlier. While these details are not central to the stories, there is a lot of environmental insights about vintage stagecraft. But you'd have to do a lot of reading to dig up this info.
Besides pages ("Callboys"), SMs and stagehands in opera historically stood on really tall ladders backstage and waved flags around for entrances/exits, blocking (ex: flags cross, parade stop CS). I believe that this method was used particularly at the Met, and perhaps even originated there. Opera is unique, too, because star singers were usually flown in at the last minute to sing a role in their standard repertoire. They certainly knew the music, but because they'd never worked on this particular set or with this particular director before, there was a very special backstage employee called the Prompter who stood DSC under the stage, poking up just enough so their eyes and mouth were visible. There was a little roof built over them so the audience couldn't even see them. The prompter mouthed the words, pointed singers towards their blocking, was ready musical cues and emotive reminders if necessary, and generally kept the stage action running smoothly. It is an amazingly complex position that actually only recently because obsolete (mid-70's?). At older houses, you can still see the prompter's box. I don't think this method was used in theatre at all, though. Anyway, the prompter was concerned exclusively with onstage action, while the SMs were waving flags to keep the backstage running.