Ach, would ye look at thon eejit, beefed to the oxters he is, the wee crathur.
Ach, bit Ulster Scots is nae sae diffren fae the Doric, efter aw, ken.
And (code-switching), there you have one thing. Not everyone comes from the same place, or class, or caste, or background. So different languages, and dialects, and also speaking the language of your story as a second language. And lots of people code-switch between languages, or dialects, and to an even greater extent between formal and informal, professional and home.
If you have involvement with the military, you pick up slang and terms of reference which only make sense to others in the same job. You wouldn't use the same phrases at home. That said, the almost cliched military bearing and clipped tone does sometimes work its way into the personality, but it's usually small things which mark them out - like a seemingly frail and elderly person standing straight and using their long-lost parade ground voice to grab the attention of a crowd, or stop a bully in their tracks.
Likewise, lots of professions have their own vocabulary. Fishing boats in the North of Scotland, and the Northern Isles, had various traditions, often for luck, such as not mentioning the word 'wife', for it was considered bad luck to talk about home. And minister, or priest, was an absolutely forbidden word. So, they had euphemisms.
Someone who works out on a hill alone may be used to taking their time in speaking, or may be so glad of the company in the evening they never shut up. Two engineers who work in the same compartment might know what the other needs almost without words, or one might speak lovingly to the engine. Work their characters into their voices and actions rather than explaining them.
I hope some of that makes sense.