What is the order that events need to be called on set, and why is it so important?
The specifics to what is called before beginning a shoot can vary from set to set and project to project, but the general goal is always identical: to get every department on the same page to prevent any problems or interruptions arising later while filming. Below is a typical scenario for what the various crew members will radio to each other before shooting a scene.
Assistant Director / Production Assistants: "Picture's up"
This lets everyone on set know that shooting is about to begin. After it is called by the AD, the PA's stationed around the set repeat it, alerting everyone in their respective area. If there is a rehearsal, however, the AD will call "Rehearsal's Up!"
Assistant Director: "Roll Sound" Production Assistants: "Rolling"
This lets the boom operators, and sound mixers know to begin recording, and for the surrounding crew members to quiet down.
Sound Guy: "Sound Speeds"
This is called to alert the directors that the sound is running smoothly.
First Assistant Camera: "Camera Speeds"
This is called to alert the set that the camera has begun filming.
Second Assistant Camera: "Mark"
The 2nd AC will slate the shot, so the cameras are synchronized in post-production.
Camera Operator / Cinematographer: "Set"
This lets the set know to be ready for filming, that the camera is focused and that the settings are correct.
Once set has been called, the director is ready to call action.