For those who are interest, here is some info on doubles and stand ins. They are not the same and each is used for different purposes.
Body doubles, including stunt doubles via Wikipedia:
A body substitute replaces the credited actor of a character in any recorded visual medium in shots where the character’s body is shown but the face is either not visible or shown indistinctly, or in shots where the image of the credited actor’s face is joined to the image of the body double’s body. Body doubles are most often used for shots involving a nude scene. More specific terms are often used in special cases; a stunt double is used for dangerous or sophisticated sequences. This is in contrast to a stand-in who replaces an actor for non-filming purposes such as scene arrangement and lighting adjustments.
Stand ins via Wikipedia:
A stand-in for film and television is a person who substitutes for the actor before filming, for technical purposes such as lighting.
Stand-ins are helpful in the initial processes of production. Lighting setup can be a slow and tedious process; during this time the actor will often be somewhere else. Stand-ins allow the director of photography to light the set, the camera department to light and focus scenes. The director will often ask stand-ins to deliver the scene dialogue (“lines”) and walk through (“blocking”) the scenes to be filmed. In this way, a good stand-in can help speed up the day’s production and is a necessary and valuable cast member on a film.