Being thrifty, I'll post it here too.
Going into the POV of a secondary character --
There are no 'rules'
(--that should be in neon somewhere--)
but you should have a good reason for going into a secondary character's head.
The good reason should be something more than just ...
-- this is an easy way to tell the reader where the cookies are hidden, or
-- neither of my POV characters are in this scene but I want to write it anyhow.
You might consider Omniscient Narrator in those cases. Or write around the problem.
I go into secondary characters' heads three times in two books.
(I think that's all.)
In two cases, this is a single excursion into their heads.
In none of the three cases does this POV choice
-- solve a plot problem or
-- convey information to the reader or
-- put us in a necessary scene we would otherwise find hard to enter.
I go into the secondary heads
to show something important about the secondary character and the way he sees the world.
In two cases, I want to put the reader 'outside' the main protagonists at a particular moment for complex reasons having to do with how the reader is emotionally connecting with the ongoing story.
When I went into the secondary POV, it was because this gave
(a) a refuge from involvement with the two protags,
(b) a new coign of vantage, and
(c) an insight into the minor character.
Talking about two scenes here ...
The scene where Galba plays chess with Annique is an example of using secondary character POV as a refuge from the two protags.
How secondary is Galba?
Galba is sooooo secondary! He is so bloody secondary he could get a medal for it. Galba appears on stage only a half dozen times, all in the last quarter of the book. If you look at him objectively, he doesn't actually do anything.
So, leaving aside Galba's insight into Annique, which is fine and wise and all that, his POV scene is not to talk about any of the characters. It's what you might call constituent. It's there to serve a structural purpose.
Look where I've put his scene.
We got a big scene of Annique betrayed, on every level, by those she loved and trusted.
Grey has to watch her hurt and he can't do anything about it.
Now we want to get on with action of the story because there's not much more to say about that emotional topic right there and, anyhow, the world hasn't stopped even though Annique is in pain.
But we don't have to skip directly from
Annique- (or Grey-) POV-in-pain to
Annique- (or Grey-) POV-getting-on-with-life
So we put in a Galba-POV to give a buffer and 'tell about' the transition period.
If I were a better writer I'd have put in a riveting scene of Annique's acceptance and recovery instead.
But I'm not. (pooh)
We could do the same buffering with a good long passage of description or something in Omniscient Narrator. But I like Galba and I'm glad to have a chance to crawl into his head.
Look where I gave us a scene of Adrian POV.
He is almost a third protag. Now contrary to what you might think, this does not make me want to fill the story up with his POV. He diverts attention from the H&H, which is not good.
So we keep his little POV scene short and simple.
This is Adrian swimming out to the smugglers' boat.
How is this constituent?
That scene falls at that halfway division in the story where everybody's crossing the Channel.
(I mean, just everybody.)
The Adrian-POV scene is a buffer between Annique's emotional experience on one side of the Channel and the other. It's there for structural reasons.
what we have in those two scenes above is what I consider a good reason for switching into secondary-character POV or Omniscient Narrator POV.
Not so we can reveal information.
Not because it's the only way we can talk about this scene.
But for structure and pacing.
This 'secondary POV-ing' is a technique that lifts you out of the protags' emotional journey and forms a buffer when you're transitioning from one emotional place to another and you, like, don't want to do it too fast.