Does a character sound like he's reading a term paper to the class?--perfect diction and grammar? Unless he's a college professor, you're probably taping word balloons next to his head. He'll sound more natural with his own style and vocabulary. Even educated types will 'phase-switch', or tailor their speech to the audience at hand. That means not being a stickler for the subjunctive mode at a family cookout. You're more likely to say 'If I was you' than 'If I were you'. Your delivery to a kindergarten group in a library would be fun and simple (kids love to laugh). Story folks can do the same. There may be a minority character who sounds straightlaced except in his own community, where he uses jargon (I play tennis with a guy who does that).
The Case For Punctuation
For the last word on the subject, see a tiny book called A Dash Of Style by noted editor Noah Lukeman. Each punctuation mark has its own chapter. Increasingly, though, you'll see things unheard of years ago. To whit: you can start a sentence with an ellipse to show hesitation. ". . . .That's insane!" You can drop the apostrophe for a narrator who chronically loses a final G: "Somethin is happenin there." The rule for hyphenated words is clarity. Some writers insert one between vowels in words like re-entry or re-invent. It's especially wise for odd constructs like 'uni-mindedness.' That one's a head scratcher without the hyphen, because 'un' can be misinterpreted as a negative. Hyphens can turn any word pair into an adjective: 'Hard-brawling miners'. The double hyphen creates a hard stop to emphasize a clause: "Whitmore stayed home--anywhere but work--until the crisis passed.' These are a few of the tools useful for helping characters stand out from the pack.