Mulling over viewpoint, I hauled Swain down from my high shelf.
Creating characters. I looked up viewpoint in the index and scanned all he had to say. Clear as day, here's where the rule originated, I suspect!
"Whatever your story, your readers will need some kind of orientation point, some place from which to watch the action. In other words, a point of view.
Ordinarily, that point is in a character - a viewpoint character. Or as I used to put it, "Whose skin are we in?" Through whose eyes are we seeing or experiencing the story?
In choosing this character, you limit yourself to presenting your story as he experiences it. That is to say, he can watch what other people do, but he can't see himself. (Jack watches others, but cannot see himself.)
He will, however, know anything you want your readers to know about his own state of mind." (Jack knows his own mind, though Janet might puzzle him)
Characters can only be observed from outside, by another character. (So Janet can see Jack and describe him to us) Swain goes on to say this limits emotional intensity. We need Jack's thoughts to go with Janet's observations to obtain that intensity (she may be inaccurate if she decides she knows what he is thinking)
To avoid this there are stories that jump around "like a frog on a hot griddle" as they leap into and out of the hearts and minds of assorted characters. This is known as author omniscient viewpoint. This is OK as long as it is done with skill but done badly can be confusing , irritating and lack the emotional intensity we're after.
There's also the objective approach - record the story entirly without going into anyone's head. He quotes Hammett and The Maltese Falcon as an example, but warns that it takes great skill and most of us don't have it.
Most fiction today is from the subjectively orientated central figure, ie it tells the story from inside the chief character's head. WE see what the character sees, hears, tastes, smells, touches, thinks, feels - we experience the story, live through it with the viewpoint character.
It grips the reader, but it limits your presentation of other characters to external only.
It means you can't lie about your central character, since you know his every thought.
Having two or three viewpoint characters is not unusual. Swain recommends a chapter for each - time enough to get used to your character - but don't let it run on so long the others are forgotten. And be sure to establish time, place circumstance and Viewpoint every time you change.
My thought is that all rules are there to be broken.