Theater work can be that way, too, though you generally have at least one read-through or rehearsal where you have some breaks and take notes prior to the actual shows. Then you have to go home, build the sounds "blind" and hope for the best when the downbeat drops.There are very few full time session guys, and they are the only people I can think of who would be literally on the clock while dialing in sounds. And not all producers expect a session guy to be able to get every guitar sound known to man in the middle of a session.
Still, though, the "time is money" mindset always exists when you're one guy in a 20-piece orchestra. They don't ever have to sit and wait for a trumpet or violin player to adjust their equipment.....