So what does a stage manager do exactly? To put it simply - we coordinate. There are many people that work together to put on a play - producers, general managers, designers, directors, actors, musicians, crew, company managers, front of house staff. The biggest part of a stage manager's job is facilitating communication between all those different people to get the show up and running.
Our job begins before rehearsals for a show begin. This is called "prep". A PSM is usually on contract a week or two prior to rehearsals, the first ASM starts the week before rehearsals (along with the PA's) and the second ASM will join the team a day or two rehearsals start. Prep consists of production meetings, notifying the actors of the first rehearsal time/place, preparing the rehearsal hall (which includes taping out the groundplan of the set on the floor), creating all kinds of paperwork from contact sheets to report templates to prop lists, shopping for office and first aid supplies, copying scripts and scores, communicating with various shops about what mock-ups are needed for rehearsal - from props to set pieces to costumes, scheduling costume fittings, ordering coffee and catering for the first rehearsal, and many other tasks as well. It is a busy, busy time - we want to make sure everything is in place to start off the rehearsal process smoothly with all possible necessities covered.

Prepping the rehearsal studio for the "Rock of Ages" tour
The studio rehearsal process for a play is usually about 3 weeks. For a musical, depending on how complicated it is, rehearsals in the studio could last 4 or more weeks. Our union contract provides us with one day off a week, which is usually Sunday or Monday. We rehearse 8 hours a day.
The stage management team arrives each day an hour before the actors. We get the room ready for the day - start the coffee, distribute the daily schedule, set up any sets/props needed for the start of rehearsal.
During the actual rehearsal process we are responsible for many things. Among them are: scheduling the day and keeping everyone to that schedule throughout the day, notating the blocking (the movement of the actors as designated by the director), tracking props, scene shifts, and costume changes (and creating paperwork that explains all this to each pertinent department), being "on book" (one stage manager or PA follows along on script and feeds lines to the actors when necessary), being the crew for scene shifts in the rehearsal room, running rehearsal sound, calling the room to order when necessary, taking notes on all the production elements created in rehearsal and then creating a report at the end of each day that is emailed to everyone involved in the production, scheduling costume fittings, and making a million copies, and brewing a similar number of pots of coffee throughout the day. At the end of the rehearsal day, the stage management team usually stays for about an hour after the actors are released. We work out the schedule for the next day with the director, create the rehearsal report to be sent out to everyone working on the show detailing what happened in the room that day, and cleaning up the studio and readying it for the next day.
As the rehearsal process gets closer and closer to tech, there will be run throughs of the show in the studio for designers and other department heads working on the show, as well as the crew and wardrobe staff. A few days before tech ("technical rehearsals" in the theater) begin, the PSM will leave the first ASM in charge, and will spend much time at the theater, where the sets, lights and sound are being loaded in, and will often begin "dry teching" the show (working out scene shifts without the actors around).

Actors on the first day of tech in Chicago for the "Rock of Ages" national tour
Once the show has been worked out in the studio, tech rehearsals begin at the theater. This is usually 3-4 weeks into the process and lasts anywhere from 1-2 weeks to 6 months (if you are working on a highly technical Cirque du Soleil type show...or "Spiderman" - ha). A pretty standard tech time is two weeks. During tech, the PSM is at a console out front working with the lighting and sound designers on all those cues, creating the original "call script" (stage managers cue all light cues, some sound cues and all scenic cues during the show, either verbally or with cue lights). The PSM Is in charge of the tech, communicating to everyone on headset and over the "god mic" where they should start and also calling "hold" when the action should stop. The ASMs are backstage, figuring out how scene shifts should work, where prop hand offs have to happen, making sure actors and crew are where they need to be at all times and managing the entire backstage "show". PAs are usually very busy during tech, for more info on effective stage management contact the phone number in the profile.