Stress
Yeah, but the good kind. It's the stress of being pushed to do what you love over the course of far more hours and under much more physically demanding conditions than you would perhaps ideally like. You may get a nice paycheck, but you'll have earned it.
There won't be much time for you to just hang around on set with nothing to do. There's always someone with a hair out of place, or someone who has a mustache coming loose, or someone whose mascara is running because they started crying after realizing they could have been something more safe and secure, like an investment banker.