An Army of One
An example of how movie making differs from other professions.
Our Script Supervisor overslept and missed the crew van from the hotel. This led everyone to freak out. We're calling her on her cell. The AD's are calling her on her cell. I'm on the phone with the front desk, bullying the GM of the hotel to personally go to her room. My assistant is sending a PA to the hotel, shoo'ing him out the door with no instructions other than 'go to the hotel, run, we'll call you with details, go, now.'
Our day literally ground to a halt while we searched for this one person because we simply cannot proceed without them. We can't shoot without a script supervisor to monitor the continuity, time the takes, record the relevant direction and log everything for the editors. It's a highly specific discipline that can take years to master so it's not like just any person can step in and fill her roll, even if the union were to allow such a thing, which they won't.
Moreover, she's not terribly unique in this regard. There are dozens of people without whom shooting simply stops and it's not just the Director, DP, and lead actors. This could happen for the DIT, Gaffer, Costume Designer, Stunt Coordinator or the 1st AD. If any one of a dozen or more people is off the mark, a crew of more than a hundred sits around on their elbows.
Make sure to set your alarm.