Characters convey their personalities through the way they speak, and the more natural the speech is, the more accessible the character. That is partly why stories and plays are not written to sound like chemistry text books. It’s called speaking in the vernacular.
Depending on the venue, grammar rules can and ought to be flexible. Avoiding split infinitives, for instance, is a rule left over from Latin where infinitives are one word, not two. Is it really going to thoroughly unravel the message if I say I need to quickly run to the store for more eggs? Of course not. And half of you probably didn’t catch the “mistakes” I just made anyway.
In casual writing, following stuffy, prescriptive rules, with all those “to whoms” and “with whiches” feels like legalese, not a blog post from a friend. Intentional disregard for a rule can create a timing or mood effect that enhances the writing. Conscious use of fragments, for example, can direct pacing or add emphasis. And it’s how people talk. (Anyone who has ever had to transcribe candid speech can tell you that. Some sentences contain more switchbacks and drop-offs than a hike in the Columbia River Gorge.)
A word of caution:
“natural” is not the same thing as “sloppy.”
It is a mistake to think that grammatical conventions are unimportant. They provide clarity. A communication world without proper punctuation gets messy and confused very quickly.
Going along with this are all the verbal cues that let a reader know whether a person is a child or an adult, highly educated or a second-grade dropout, even American, English or Australian (yes, we write with an accent).
My favorite way to work on this skill is to listen to real people talking–whether it’s unscripted reality shows, old folks hanging out in the doughnut shop, or teens at the mall. I listen and learn, and then try to apply what I hear to what I write.
I’m not trying to impress anyone with the words I choose. I’m hoping the appropriate vernacular will show!