punctuation - Is it acceptable to punctuate and capitalize a dialogue tag like a separate sentence?

When a piece of writing has a quoted sentence followed by a dialogue tag, the sentence is usually ended with a comma (inside the quotation marks) and the dialogue tag is in sentence case, like this:

"Look," said Hermione.

"I never pegged you for a fan of the obvious, Sam," she said.

The above is the style recommended by, say, the page 8 Essential Rules for Punctuating Dialogue.

On a couple of occasions, though, I've seen people writing the quoted sentence with a period at the end, and then starting the dialogue tag with a capital letter, like this:

"Hey." Said John.

"Not a problem, buddy." Responds Keith.

Is the latter style acceptable too? My best guess is no, it's not, simply because I've never seen it in print, and it makes it seem like "Said John." and "Responds Keith." are supposed to be complete, independent sentences.

Answer

The conventions in prose have the purpose of making the writing invisible. Prose writers all use the same convention to mark up dialogue because that makes it easier for readers to forget that they are reading and immerse themselves in the narrative. That is why prose is commonly printed in an unobtrusive typographic font, written in the standard language (not dialect or slang), and why all writers follow the same orthographic rules that readers are taught in school.

Writers (of prose) do deviate from these conventions only if and when they aim for a specific effect, because every deviation from convention will irriate the reader, break their immersion, and detract from the narrative.

So while you may of course punctuate your dialogues in any way that you want, a professional writer will always use the common convention unless he specifically wants to estrange the reader, for example because he wants the reader to get the impression that the book was written by someone whose thinking deviates in some way from the norm, like a narrative by a robot or someone with autism.

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A single case of "Hey." Said John. within an otherwise conventionally punctuated dialogue might mean that the narrator pauses before he gives the dialogue label:

"Hey." (Pause.) Said John.

Here is a full example for how this might be employed:

Unwillingly, Peter took up the book. "Hey. Said John," he read.
"Please don't pause between hey and said John, Peter," admonished Mrs. Greenwell.
"Hey said John," Peter read.
Mrs. Greenwell sighed. "Well, you do need to make a slight pause, of course. Please don't play dumb with me, Peter."