This might be a kinda silly/obvious question to some, but what is the correct way to use 'sir'? I have a character who generally refers to another character as sir to denote the class difference between them, but I don't really know how often they are supposed to say it when talking to them. Are there rules for when it is and is not required? It feels awkward to put a 'sir' in every time they say something to the other character, but I don't know when it can be omitted while still seeming proper and polite.
It is a polite way of showing respect, though once it is established you don't need to rub it in. Though I write Mil-Sci-fi so it is common to refer to someone as Sir. However I would say in your case I would limit it to greetings, and when they thank them for something. That way you aren't using it constantly. Hope this helps a smidgen.
When I was in school I generally had a difficult time remembering the names of all my teachers, so I basically referred to them all as either Sir, Ma'am, or Miss. So anytime it would seem natural for a character to use the higher ranking characters name, 'Sir' could easily be substituted and to me it would not seem out of place. Using it more could indicate a more strained relationship between the characters and less a more relaxed, familiar relationship.
I was an officer in the Army. As a common occurrence, I said "sir" all of the time, but as someone earlier stated, it can be overwhelming after some time. So what I used to do was say it at the start, and then throughout the rest of the conversation, it would be used sparingly. Eventually, you get used to the number of times to bring it up, and one of the other respect techniques I would use is to address the higher ranking officer by that person's rank every now and then instead of saying sir. But as the conversation would go on, I'd tend to stop using it until I hit the end of the conversation. I do remember that when I was at West Point, I was sometimes accosted by higher ranking cadets who wanted to hear it ALL THE TIME, and sometimes they wouldn't know when it was appropriate so they'd have you say it at the end of the sentence, even if you used it at the start of the sentence. So, they were wrong in wanting this, but when someone's the higher ranking one, they tend to get their choice in such matters.
