Each time you stand in, you feel out just how immediately you should step in. Sometimes the crew needs to move set pieces before it is even safe for you to step in. Whatever the case, when first team steps off, you are ready to be on your mark should the director, DP, or ADs need you.
When you step to your first position, you line your toes up flush with the mark. Your actor has a specific tape color, so as you move to your second position and later positions, you hit the marks matching your actor’s color. As you are asked to shift off of a mark into a new mark, a camera assistant will re-mark your position. You don’t re-mark yourself.
While the crew sets up a shot, the DP will look at you, observing how the light and shadow hit your face, perhaps clicking a light meter very close to your face to get a light reading. The camera operator will frame you and ask you to move from first position to second position and so on. A camera assistant may walk up to you and take a measurement near your eye, or the camera assistant may send a tape measure out to you and have you hold it at the side of your eye. At times it may seem as if you’re not being used, but you won’t always know when you’re being used. Often, you are being used without knowing it!
Because of these factors, when you are working, ideally you are silent, still, and focused. There is a lot of chaos swirling around you that can easily distract you as you stand in. Although you may be tempted to chitchat with the stand-in working next to you, it may get in the way of your concentration.