Point of view. It sounds so wonderfully straightforward, doesn't it? Pick a character. Tell the story from their point of view. Simple!
We then get sentences like "She turned purple with rage." What's wrong with it? It kinda violates POV. The last two words specifically. See, if I am the POV character, I can only tell you what I observe. I observed that she turned purple. 'With rage' is an assumption. IF that assumption is being made because it is within the confines of my character to do so, then okay - writer using a character's jumping to conclusion tic as a device within the story - maybe altering that character trait is part of the arc. Roger that.
It's just that 99% of the time, that's not what's happening. Certainly, it isn't what's happening when *I* do it. No. Most of the time, it's lazy story telling. Shorthand that let's me pretend there's emotion in my scene. Except that using my POV character to tell you someone turned purple with rage is just that - telling you. It's shallow POV. Showing is easier if I go deeper into my POV character's head, which makes it harder for me, as the author, to intrude. Then the same observation would look more like this:
Air hissed in between her clenched teeth. Her face flushed purple and she clenched her fists. I backed up a step.
Not a single feeling word in there - just a report of what a POV character saw, heard, and then how she reacted. From those things, you have a good grip on the underlying emotion. Given dialogue and context, you'd know whether my POV character had just delivered bad news, challenged this other gal to a duel, or what have you. You'll notice I automatically put this in first person. It's easier for me to get deep POV if I draft a story in first person. Most of my stories end up being 3rd person and I make that change in rewrites. Makes for interesting typos but drafting in first reminds me to stay honest about what my character can conceivably see, hear, smell, taste, think or feel.
POV is about accepting the constraints and limitations of only knowing what a single character can know. That's why so many stories trade off POV characters between scenes and/or chapters, so you can get more than one perspective on the central conflict. When I do that, generally sharing story telling between my heroine and hero, I have to decide who should have the story telling reins, when. I ask who among the characters has the most to lose. Sounds simple. Rarely is. It usually means me writing a scene or chapter both ways because I'm pathetic like that. Don't get me wrong. Usually, the POV I write in the first time (THIS character has the most to lose) is correct, but second guessing R I, thus the whole thing gets done from the other POV, too. Just in case *that* one is better. Leading my own observation that Point of View is certainly craft first, but once the mechanics are worked out, it becomes an art in your hands - one that dictates the voice and color of your stories.