A writer is accused of head-hopping when the thoughts of more than one character are shown without a break of some kind. It’s considered sloppy at best.
As writers, we do our best to have the reader identify with the protagonist. Most of the time, that is also our main viewpoint character. This is who the reader spends the most time with in a book, and the writer always hopes the readers want more. It is also our first rule to be clear.
I know, there are a ton of big-name authors who don’t follow those rules. The thing is, they’re big-name authors and people buy their books just because they’ve been written. I’m not that lucky. While there are a few people who have purchased my book that I don’t know, the majority of people who own The Art of Science I know personally. (Thank you, everyone!) I want to write well enough to become a big-name sometime. At that point, I hope I do still play by the “rules” of writing.
No promises, though.
Clarity begs us to write from one perspective at a time, like we see as we go through life. It makes things clearer to know, for certain, that you will only be behind one person’s eyes at a time.
I mean, when was the last time you were sitting in a classroom and actually heard the teacher’s thoughts? You can evaluate his demeanor, find positive or negative qualities in the tone of his voice, or interpret his gestures in any way you want, but part of that will be based on the teacher and the rest filtered through some part of the viewpoint character in the way conclusions are drawn from there. And if not through the viewpoint character, then by intrusion of the author.
Giving direct thoughts that can’t be heard by the current viewpoint character is the author telling us things, rather than showing how it happens. Unless, of course, you have an actual mind-reader.