We have to distinguish between the dialect of English called the “formal writing style” which is what the style guides act as prescriptions for, and the actual (various) living versions of the English language.
The formal written language that you are taught in school is not a real living language. If you were well-educated, it may seem to be identical with the language you use every day, but it isn't. It's a status marker for “educated” and appropriate for non-fiction which is intended to be formal, but it's not a proper guide to writing fiction which should capture living languages, not artificial ones. It's not a matter of vocabulary choices (though that plays into formal/informal distinctions) — it's a matter of the actual language itself and its grammatical structures.
The formal written language is, um, written down and its rules change slowly and rarely. The natural language continues to evolve constantly and, as writers, it's the natural language we should be concerned with. (Or none of us would end sentences with prepositions or split an infinitive on the once-fashionable theory that Germanic English is somehow Latin.)
For example, just as in circa-Elizabethan times we saw the loss of 2nd person singular personal pronouns (thee, thou, thy, thine) to the expansion of 2nd person plural (you, yours), we are currently living through a similar evolution in the language with regard to gendered 3rd person singular personal pronouns, where (he, she, him, her, his, hers) are being replaced by the ungendered 3rd person plural pronouns (they, them, their, theirs) in gender-neutral situations, as the clumsiness of using the male 3rd person singular as a stand-in for unknown-gender is being eaten away by a disregard of number to solve the problem.
It now sounds perfectly normal even for the well-educated to say something like, “If anyone comes in early, give them a drink.” That wasn't true a few decades ago.
We are drowned in new vocabulary, slang, and idiom constantly, but it's unusual to be able to actually see a grammatical change this large in a human lifetime. We know this isn't proper for the written language, but we're no longer willing to say “him or her” or just “him” in our natural language use in that situation, even though that's what the formal written language still requires.
All of this is a long-winded way of saying use a style guide for guidance in punctuation (it's the written language, after all) and to convey conventional high-status artificial language for formal writing (or your readers won't respect you), but use the living language for fiction and especially for dialogue.