Clarification/Follow-up by denberg on 01/26/04 4:51 am:
You said in your answer that the biggest part of a stand in's job was to stand in the position that a principal character would be in...what other jobs would he have? I thought that was it!

Clarification/Follow-up by voiceguy2000 on 01/26/04 9:03 am:
That really is about all the stand-in will be doing, especially on a union shoot, where most other tasks on the set are within the jurisdiction of other crafts. The double may occasionally be used as a background actor (extra), and in some cases may communicate to the principal certain information that had come up while the next scene was being set up. He might be drafted for certain gofer tasks that do not intrude on any union jurisdiction or interfere with his ready availability to do stand in the moment he is needed for that.

My understanding is that certain actors (Nicholas Cage and Bruce Willis being examples) have favored stand-ins that they request for each of their films. I would expect in such cases that the stand-in may also become something like a personal assistant to the actor.

Mostly, however, the stand-ins get to hang out on the set for 10, 12, 15 hours per shooting day. It is their job to take the fatigue that the actors would otherwise have as each scene is reset for master shots, close-ups, etc.