Differentiating comma/period use in mid-dialogue

Solution 1:

As I understand things, you place a comma after the interruption if the first quotation was not a complete sentence, and a period if it was. The rule really is as simple as you inferred. The stylistic element comes in where you have to decide how you would punctuate a quotation if it wasn't to be broken.

By example:

"My most worthy adversary, we meet again," becomes, " 'My most worthy adversary,' the villain scoffed, [comma] 'we meet again.' "

Whereas

"My most worthy adversary. We meet again," becomes, " 'My most worthy adversary,' the villain scoffed. [period] 'We meet again.' "

Solution 2:

You would place a comma after the interruption if the first quotation was not a complete sentence, but dialogue doesn't have to use complete sentences. Using the period in sentence 1 amounts to deciding not to use complete sentences within the dialogue.