Two deliberate sentences in dialogue

rich_cheese said:

"Nakamura." she said. "You know why we're here."

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The example is incorrect for the somewhat baffling reason that it's not permitted to use a full stop before the tag, even though it's perfectly acceptable to use an exclamation mark or question mark, both of which usually function in the same way as a period (i.e. they end a sentence, although of course they have different nuances):

Incorrect: "Nakamura." she said. "You know why we're here."
Correct: "Nakamura!" she said. "You know why we're here."
Correct: "Nakamura?" she said. "You know why we're here."
Correct: "Nakamura," she said. "You know why we're here."
Correct: "Nakamura." She paused. "You know why we're here."
Correct: "Nakamura," she said, "you know why we're here."

I'd probably use the first comma option. Someone else might provide a better alternative.

Instead of an alternative dialogue tag, perhaps you should split the two statements with a line of short action.

"Nakamura." She inclined her head - a mock salutation. "You know why we're here."

Or some such.

I don't think it's "wrong" the way you have it, though.

rich_cheese said:

From what I understand, if I had:

"Nakamura." she said.

on it's own, it's correct.

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Though that's a reasonable assumption, it's just not done. Check out the dialogue thread on this point.

rich_cheese said:

From what I understand, if I had:

"Nakamura." she said.

on it's own, it's correct. Maybe I don't fully understand the grammar rules for the next part, but maybe I'll just bite the bullet and put a comma in it.

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~~Ah, well, no. 1) It's 'Nakamura(COMMA), she said. ~~the conventions do not permit the use of the period while permitting the question mark and exclamation point, which are also full stops. It's not logical, it's just the way it is. 2) It's not it's own, it's its own. It's correct is correct. 'Its' is like 'his', a possessive pronoun. 'It's', sometimes formerly written 'it 's', with the space between the two words, is a contraction, and the apostrophe shows where the omitted 'i' went. It does not indicate the possessive form.
Despite what some texts say, punctuation does NOT indicate pauses. You have to do that manually.

You can't use a period before a non-capitalized letter. Doing so doesn't add length to the pause, anyway. The pause is exactly the same with a comma. It's the "she said" that gives length.

Just punctuate it correctly and write, "Nakamura," she said. "You know why we're here."

It reads fine now the punctuation is correct

rich_cheese said:

And that's what I'm doing. Just wanted to clarify that, not anything else. Thanks.

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rich_cheese said:

From what I understand, if I had:

"Nakamura." she said.

!

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This isn't correct, either. Even on its own, it must use a comma.

In this case I would use a short action tag to make sure the reader understands the pause.

"Nakamura," she said. "You know why we're here."

Seems to me that's perfect. You don't have to say there's a pause; we feel the pause. That's one use of a dialogue tag.