Teaching somebody to fall can be nerve wracking. I've taught some guys and, mostly due to their good attitude, they came out alright. But you still wonder sometimes if you told them enough and showed them enough.
What you said about "common sense" is true. That's half the job. Learning something from your mistakes is key, too.
There are less and less young guys that want to be fallers. And less jobs for them, too. With so much mechanical cutting we always seem to have an over-supply of fallers around here and some guys have left the business altogether. A guy with some skills, a good reputation, and some connections can still make a pretty good living but a guy just starting out will have a hard go of it.
If you hire a faller you'll need to remember that they'll never ever do it just exactly the same way you do. That's not necessarily bad and if their results are good and they're doing the job you just have to let them go about things their own way. It's hard to do, though.