Edward Dmytryk’s 5th Rule of Editing: All scenes should begin, and end, with continuing action
Scenes should not start at the moment the actor starts his first action for the scene. Scenes should not end at the conclusion of the last action of the scene. The audience needs to get the impression that each scene is ‘a fragment of continuing life.’
This is a matter for experienced directors to allow for warm-up actions before the action at the start of a scene, and for editors to recognise these actions so that they know at what point in time to start. The examples Dmytryk gives are an initial walk across a set, sitting down or standing up or hanging up a telephone. Actors can use these actions to ‘get up to speed’ so that the context is set for the scripted content of the scene.
Extra actions at the end of a scene also help editors cut while the actor is in action, so that the audience is left without a complete resolution, a resolution they hope to find in the next scene.