Here's the Hand-Off...
Let's say that you've decided to use close third-person narrative to tell your story. Except you want the freedom of changing viewpoint characters from time to time within the book. How in the world do you manage the transitions? How does one VP character trade off to the next?
There are many ways, some homely and straightforward. Some writers simply label passages with the names of the viewpoint characters, changing where indicated:
The wind blew cold, making him tuck his neck down and pull his head into the turtleneck of his sweater. The knife-edge of the wind nearly cut tears from his eyes.
There he was at last, Stephen, walking fast. Was he going to pass right by her without recognition?
Doesn't work for very short passages, but for scenes* or chapters* it can be quite unobtrusive. Other writers don't give character labels, but they do make some obvious reference early in a new viewpoint section so the reader doesn't feel lost.
What a crummy day. Stephen closed the apartment door behind him, wondering again what was wrong with Mary, anyway.
- You always have to be clear about the focus character yourself. With close third person, once you're in, you're in until you reach a natural transfer point to hand off the focus to another character;
- Your characters ought to have personal voices so that, even without a label, their habitual way of thinking, of phrasing, of seeing the world, identifies each point of view as distinctive and different from all the others.
*Scene - A scene is an arc of continuous action, narrated with attention to setting, character, action, dialogue, and detail. It may last only a paragraph or two, but more often scenes run about 500-1000 words.
*Chapter - A logical collection of scenes, length flexible. Sometimes a very brief chapter will be one scene long; more often, a chapter is from three to ten scenes long. Like a scene, a chapter should be something of a unit, making its own point and carrying the action forward to what seems like a natural point of rest before the next section picks it up again. It is possible to write an entire book consisting only of scenes, with no chapters. See most of Terry Pratchett's Discworld books as an example.