Sorry, meant to add this:
“Must of” is a writer’s mistake, not a speaker’s mistake. So it’s as if the author is turning his character into a writer instead of letting him just be a speaker.
The mistake the writer is making is that they’re not transcribing the speaker saying “must have”, they’re mistranscribing the speaker using the contraction, “must’ve”. The speaker isn’t saying anything funny or incorrect - “must’ve” is perfectly legitimate.
If the writer is trying to make a point of the character’s pronunciation, he would be better off writing “musta”, which would immediately stand out as showing pronunciation. “Must of” merely stands out as being incorrect.
ETA: Curiously, I’ve been working my way through Terry Pratchett’s Discworld novels, and I’ve spotted numerous instances where he uses the incorrect “should of, would of, could of” construct. Given Pterry’s otherwise brilliant command of the language, I have to believe he’s doing it deliberately to make some subtle point.
My Goodreads review of a book:
“It’s pretty good. But,” she said, “the author uses this device all the time.”
“What,” he asked, “device do you mean?”
“The device,” she replied, “of splitting dialogue around stage directions.”
“Oh, that,” he remarked, “is so annoying.”
“Yes, it,” she answered, “is.”
:smack: Doh! My bad, I didn’t even think of it that way; you’re right.
I’m pretty sure the narrative term “stage direction” applies instead to the examples that olivesmarch4th was using upthread: non-speech actions used to attribute who’s speaking in lieu of an explicit “said” or equivalent tag. The examples you’re depicting here are called “mid-speech dialogue tags”. Your punctuation is technically correct, but the tag placement in most of them is horribly inappropriate, even if you consider each one in isolation without the overuse problem. 
Indeed.
I quite like that style of writing, FWIW…
A mid-speech tag strongly implies a pause in the speech to the reader, usually one longer than a comma indicates. Jsgoddess’s examples make the speakers sound weird because most of the spots she chose are unnatural places to pause while speaking. You usually only want to use it where it’s okay to punctuate with a comma. Anywhere else, you call attention to the pause.
It bugs me when the writer overstates the obvious:
“No,” I lied. [It’s probably already been made clear that the person speaking is being dishonest, so there is no reason for the word “lied,” and it sounds clumsy anyway.) Stephanie Meyer does this all the time.
I also don’t like it when the writer has characters repeating what another one just said:
“We have to go to Paris right now!”
“We have to go to Paris? Now?”
Dan Brown is guilty of this. I started thinking everyone in The DaVinci Code was hearing-impaired.
Meyer probably would have written “‘No,’ I lied dishonestly.” That woman never met an adverb she didn’t like, redundancy be damned. I remember she did at one point in Twilight write “he whispered quietly.” In case maybe the reader got confused and thought it was the loud kind of whispering. :rolleyes: