Bsrgvty

You're taking "show, don't tell" too strictly. There's no rules in writing - they're more what you'd call guidelines.

If you're in doubt about a passage, write it both ways. Then see which one feels more natural, and which one feels weird and convoluted. If you're finding yourself writing in an unnatural and convoluted manner just to follow some rule, don't follow the rule.

In particular, things that your POV character already knows about another character, or about the world, or things that everybody knows - it is very common that those be told. What you're showing then is that your POV character knows something, and acts with regards to that information.

"Show, don't tell" has a rationale. If you show nothing at all, if your whole story is written in drab monotone, there's little for me to connect to. If you're telling me one thing, but showing another, for example you're telling me that a character is courageous, but he's never showing that quality, and in fact bravely runs away away, that can create a disconnect. But this doesn't mean you have to show everything.

The "show, don't tell" idea comes from cinema, where you can indeed show. Literature is a medium of telling a story. You want to tell it vividly, you want it all to appear in your reader's mind, as if they were there, and the rain were falling on their faces, and an eerie scythe-like moon were lighting the path in front. But in the end, you're still telling.

Agree with Galastel's answer, most writing "rules" are just guides so you understand the general effect on the reader.

However, just picking the one that "feels natural" isn't very objective, so I'll try to be specific how each version changes the effect on the reader.

Breaking the "rule" is sometimes how you signal subtext that changes the meaning.

Visa squared his shoulders, Reino respected confidence.

The unspoken subtext is that Reino is influenced by appearances. He is willing to follow someone who simply fakes it with confidence. Visa's pretty sure about Reino. As a reader, I am not questioning it.

(See Jane Austen's Free Indirect Speech for how to reveal character bias when their opinion is stated as a "fact" by the narrator.)

He squared his shoulders. This'll get him to listen to me. He respects
confidence.

In this case, you draw attention that this is just Visa's opinion of Reino. You are signaling to the reader that Visa might be wrong, or at the very least that Visa is self-consciously trying to manipulate Reino. The subtext shifts, and I am more suspicious of Visa than Reino.

He squared his shoulders. Reino's forehead wrinkled as his eyebrows
rose, looking at his apprentice's new posture.

I honestly get nothing from this passage. Body parts are moving, but you are not telling me what it means so it is cold and abstract. I am sort of forgetting it as I read because it feels like modern dance choreography that doesn't connect to any clear emotions or meaning.

Beware of filter words where the reader observes an observer. It creates emotional distance between the reader and the characters.

Showing actions without telling us their meaning works when the character's motions are unambiguous, and their emotions are deliberately being obscured:

She left her unfinished drink on the bar and quickly walked away. She didn't look back.

Here the character's motions are unambiguous. We don't know exactly what she's thinking because she doesn't want other characters to know either. The reader has a similar experience to any observer in the scene. We infer her emotions from her actions.

I would add one word to that (then I'll tell you why):

Visa squared his shoulders, knowing Reino respected confidence.

An opinion is part of somebody's internal life; and how they see the world.

Without the word 'knowing', it is the narrator telling us something about Reino's internal life, and we have to guess that Visa knows this.

By adding the word 'knowing', the narrator is telling us something about Visa's internal life and why he is squaring his shoulders, because he believes something about Reino. (and I am presuming from the context that Visa is the POV character.)

In writing it is okay for the narrator to reveal inner thoughts of the POV character for a given scene, that can be done as direct thoughts (in italics), or direct dialogue with other characters, or indirectly in prose as a description of thoughts that isn't in a verbatim thought-by-thought form. Or a mix of the two, for example:

Britney walked half an hour, thinking over her conversation with Tomason, and near the first mile marker she realized something: Tomason was not lying to protect himself, it was to protect Jack! All the lies protected Jack. she stopped walking. Why would he protect Jack?

If you are talking about somebody's opinion, be sure to attribute it to them.

This can be in thoughts, but remember (for the sake of realism) that most opinions are not reviewed in detail by the people that hold them, and are almost never reasoned out in thoughts, they come in snippets as they apply to a situation. Again, as above, the reason Visa squares his shoulders is he believes it will help to show confidence. We don't need to learn why he believes this and who taught him that and when and where and how many times he has used it, or what happened to make him believe that Reino respects it -- We just need to know this is a belief he holds or something he knows.

I'll add to Galastel answer about the general limits of the "show don't tell" paradigm. You are indeed allowed to tell when its more natural to do so; the catch being that while there are guidelines (and we'll be more than happy to give you some) the ultimate decision on what feels better is up to you.

In your particular case, this

He [Visa] squared his shoulders, Reino respected confidence

works better than this

He squared his shoulders. Reino's forehead wrinkled as his eyebrows rose, looking at his apprentice's new posture.

This by the simple fact that the PoV is following Visa. Visa knows something about Reino and acts accordingly. So, you're telling something about Visa's beliefs rather than actual truth. Sure, you could go into a lengthy explanation or a flashback, showing how Visa knows that Reino respects confidence ... but would it serve your narrative?
Probably not.

Showing and telling are tools, not different from a first person narrator opposed to a third person one. You have to keep them in the toolbox, but deciding which one to pick in a given situation is up to you.

All writing in print is (technically) telling. You can "show" in a movie or a play, but everything you're doing in a book is telling, if you want to get technical about it. A lot of times it is better to go with the more immersive choice, the one that puts you more in the head and the experience of the character, but not always.

In this case, one option is:

Visa squared his shoulders, knowing that Reino respected confidence.

You're "showing" Visa's actions, and telling his state of mind, and that's okay.

Another option is:

Visa squared his shoulders, and did his best to look confident in front of Reino.

This is a mixture of telling and showing about Visa's actions.

Another option is:

Visa squared his shoulders and puffed up his chest.

That's all "showing." It might be better, but isn't necessarily. You lose the direct info about him trying to impress his master, but you can arguably pick it up from context.

As you say yourself, "show don't tell" is a guideline and not an absolute rule.

I write computer software for a living. (Sadly, I make way too little money writing to live on.) I've had many occasions in the software business where someone will tell me that something I did is wrong because it violates some rule they read in a book about how to write software. And we often end up having a conversation that goes like this:

Me: "Yes, that's a good general rule, but in this case it causes all sorts of problems."

Pedant: "No, you don't understand. It's a rule. It says so right in this book."

Me: "I don't doubt that someone wrote it in a book. And in general it's a good rule. But in this particular case, there are circumstances that make it so it causes more problems that it solves." I may give specifics at this point.

Pedant: "But it's the rule. See, look, here's the book. It's right there on page 192. That's the rule."

Etc., until I either give up and ignore him or, if he's my boss, I sigh and do it his way.

If my point is not clear (because I'm trying to show rather than tell :-), just because somebody wrote a rule in a book doesn't make it absolute and unquestionable. Unless the person who wrote it is God or someone who has the power to penalize you for breaking it.

Same thing here. Frankly, as rules go, "show don't tell" is one I'd take very non-commitally.

This case is a great example. You can easily write, "Reino respected his confidence." It's short and to the point. The reader likely gets the point immediately. It's clear and unambiguous.

How would you "show" this? How would you describe Reino's behavior in a way that makes it clear that he respects Visa because of his confidence? You could write, "Reino looked at Visa with a look that showed respect." But that's still "telling". You didn't describe the look, you just said it showed respect. You could have Visa say something and Reino indicates that he accepts his words, when previously he had disputed everything Visa said, but this would add a potentially long conversation and not necessarily be clear.

Maybe a sufficiently creative writer could come up with some way to present this that made it clear. But I wouldn't even bother thinking about it. The obvious solution is to just say it. Unless there was some problem with that solution, I don't see any reason to bang your head against the wall trying to come up with a hard way to accomplish something when you already know an easy way just to conform to some "rule".

Now if there was some reason why you DON'T want to make it immediately obvious to the reader that Reino now respects Visa, that's different. Then you don't want to flat out say it, but give clues and let the reader figure it out. Or if this was some crucial plot point and you don't want it just revealed in one sentence, but you need a build up. Etc.

He [Visa] squared his shoulders, Reino respected confidence.

On its own, I take that line to be free indirect speech, and the sentence means that Visa thinks that Reino likes confidence. (He may or may not be correct; the reader can decide)

So the same thing as the third sentence, without the italics, and without literally verbalizing the character's thoughts.