Usually, but not always, the stand-in is approximately the principal actor’s height, coloring, and body type. In many cases the stand-in even resembles the principal actor.
On a TV/film set, stand-ins are considered part of second team. Principal actors are considered part of first team.
When first team steps off set, second team steps in to replace them. When first team arrives on set, second team steps off or is excused very soon after.
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download The Stand-In Handbook and The Callsheet Cheatsheet.
For a complete overview of the job of the stand-in and callsheet terminology,