Many years ago (the early 1990s), I read a synopsis of a paper written about the communication habits of people attending a science fiction convention (back when such conventions were at least as much about written SF/F as about movies/TV). The woman who wrote the paper was not a science fiction fan herself, and she was surprised to observe that many of the convention-goers tended to, as she put it, speak written English: they used very correct grammar and big words, they over-enunciated, and sometimes they pronounced words based on spelling rather than how such words are normally spoken.
Aha! I thought. This is what I’ve been trying to find a way to explain…
When I was a kid, I thought that the reason I “talked funny” was that people learn their speech patterns from whomever they’re around the most, and I spent far more time with books than with humans. (I’ve gotten “better” since then, due to years of deliberately developing protective camouflage. I still don’t have an accent “appropriate for my socioeconomic status,” though; I still say you don’t have any — or worse yet, you haven’t got any — instead of y’ain’t gut’nuh.) Even learned use of incomplete sentences from reading fiction by authors who knew how to make that work.
So… I spoke written English. Still do, to some extent. I use the word ubiquitous in normal conversation (blame The Muppet Show for that), say different from instead of different than or different to (’cause I understand what the word different means), and sometimes even pronounce all the syllables in medieval (and get angry at people who think it’s mid-evil).
What I see these days (it probably always happened, but I didn’t start noticing it until it became relevant to my career) is that a lot of writers, especially newbies, write spoken English. This is not necessarily a bad thing in small doses. Writing is supposed to sound natural (by whatever definition of natural is in vogue on any given day), especially dialogue, and most people don’t use perfectly correct grammar all the time. However, writing spoken English becomes a problem when, for example, a novelist writes could of instead of could have. “My characters aren’t grammar nazis and don’t care how ugly college-educated people spell it,” is not a valid reason to write could of. Unless you’ve written a personal diary or other actual writing by the grammar-challenged character, spell words the way they’re spelled, not how the character thinks they’re spelled. Don’t use garbled grammar unless it’s in dialogue (or first-person narration) from a character who speaks that way. You’re not the POV character; your job as the writer is to be comprehensible.