In addition to being a writer, I'm also an editor. In the last few books I've edited or copyedited, I've noticed that the biggest grammar/punctuation issue newer authors struggle with is how to punctuate dialog tags. I started out thinking of how to write a "how-to" lesson, but then I thought, why learn it from me when you can learn it from some of my favorite authors? So here are snippets of dialog from some of my favorite books. Enjoy.
Harry Potter and the Prisoner of Azkaban by J.K. Rowling:
"You know, I reckon Ron was right about you," Harry told Crookshanks suspiciously. "There are plenty of mice around this place, go and chase them. Go on," he added, nudging Crookshanks down the spiral staircase with his foot, "leave Scabbers alone."
The bolding is mine. This is a perfect example of how to handle dialog tags in the middle of sentences. Because what comes before the first dialog tag is a complete sentence, there is a period after the tag (after the word suspiciously) and the continuing dialog starts with a capital letter. But the second dialog tag comes smack in the middle of a sentence, so there is a comma after foot, not a period, and the dialog continues with a lower case "leave."
Note that these are not action beats. Although some action is incorporated when Harry gives Crookshanks that gentle kick, the primary verb in the dialog tag is added. In other words, 'he added, nudging Crookshanks down the spiral staircase with his foot' is not a complete sentence. Otherwise we'd get different punctuation, as in this example:
Thud by Terry Pratchett:
Carrot nodded. "You have to understand about a dwarf mine."
If action is used instead of a dialog tag, and the action is a complete sentence, it ends in a period. It's a sentence. Period. Carrot nodded, "You have to understand ..." would be incorrect.
Another example from Thud, where there are more than two people conversing:
"Then get someone else to do it, dear," said Sybil.
"Can I do that?" said Vimes.
"Yes, sir," said Carrot. "You're in charge."
The point here is to use said whenever possible.
Terry Pratchett and J.K. Rowling are British. Stephen King is American, and his use of dialog tags strikes me as very American. Lots of action and very, very few adverbs. Because he writes horror, he also shows a lot of emotion in his dialog tags.
Desperation by Stephen King:
She turned to Ralph and spread her hands. "The obsessive Bible reading was bad enough, but this ... why didn't you tell me about this praying business?"
"Because it looked private." He shrugged, not meeting her eyes. "And it wasn't hurting anybody."
Note that each action beat used to indicate who's speaking is a complete sentence that ends in a period. In the last line, the action comes in the middle of Ralph's speech. Yet each and every sentence, even the first line of his dialog, is a complete sentence that ends in a period. Really, try to avoid shoving an action beat in the middle of a dialog sentence. Action beats are complete sentences, and splicing a complete sentence inside another complete sentence is punctuation anarchy.
So, if you ever want to know more about dialog tags, pick up your favorite novels. Just try not to get lost in them and remember to get back to your own writing.