Stress
Four violins are making up the string section of a tune that a famous artist is paying mega bucks to book. One of the strings is out of tune and screwing up. Do you really want to be that player?
No, you don't.
This can be a very stressful job. It's not like you're a member of the band and they're stuck with you. If you screw up your part or show up late—even if it's only by five minutes—you can find yourself on the other side of a slammed door. If a gig runs long and you have to stay an extra few hours, you'll just have to deal with it—even if you have another appointment waiting.
You must be able to take all the stress in stride; you have to be level-headed, committed to your work, and even-tempered above all things.