I asserted (or maybe "said") before that dialogue tags are a convention for accomplishing one thing, namely pointing to the speaker. But the tags themselves, no matter how invisible to the reading eye, have a rhythm in the reader's ear. Rhythm in dialogue is not only a rhythm of beats, but, for lack of a better term, a rhythm of meaning. This is an element of prosody that comes naturally to many writers, but only with experience to others.
There are important rhythmical differences between
"I want them and I intend to have them," Lizzie said.
Lizzie said, "I want them and I intend to have them."
"I want them," Lizzie said, "and I intend to have them."
Because prose, like music, is a linear modality, whatever comes last gets a slight, undeserved (and sometimes unwanted) emphasis. In the first example, we get the dialogue, followed by the tag, which throws the emphasis very slightly to Lizzie rather than to what she said. In the second example, we reverse this, giving the reader Lizzie first and then throwing the emphasis onto what she said, which tends to sound a bit more dramatic.
The third example, of course, is the most dramatic of the three, because it temporarily suspends the dialogue (indeed, suspends meaning), like an actor pausing for a beat in delivery. The suspense throws the emphasis heavily to the last part of the line. The first example imparts a more matter-of-fact tone; the third sounds as though Lizzie is really making a point.
The beat can be added even more effectively (and sometimes overdramatically) by what actors call "business":
"I want them." Lizzie turned away and crossed her arms. "And I intend to have them."
It is also legitimate to insert business through em-dashes, which break the dialogue without inserting a true tag, but let the sentence run without a period:
"I want them—" Lizzie turned away and crossed her arms—"and I intend to have them."
The convention on how this is punctuated is still under debate; you also see
"I want them"—Lizzie turned away and crossed her arms "—and I intend to have them."
and several other combinations. I happen to like the em-dash interruption, but it has had a pernicious effect on how dialogue is written, leading some writers to believe they can write
"I want them," Lizzie turned away and crossed her arms, "and I intend to have them."
That's a comma splice, as is
"I want them," Lizzie smiled, "and I intend to have them."
which, to compound the sin, has her "smiling" her words.
Some writers don't seem to realize that a full stop in the middle of dialogue is legitimate. So instead of
"I want them." Lizzie turned away and crossed her arms. "And I intend to have them. And if anyone wants to fight about it, they can see me in court." She glanced back over her shoulder with a feral glaze to her eyes. "Or, perhaps they'd like to meet me in single combat?"
"I want them," Lizzie turned away and crossed her arms, "and I intend to have them. And if anyone wants to fight about it, they can see me in court," she glanced back over her shoulder with a feral glaze to her eyes, "or, perhaps they'd like to meet me in single combat?"
I have no intention of meeting Lizzie in in combat, since she's beginning to seem a bit daffy (and I have no clue what it is she wants, though she's pretty damned insistent), but the second version of the paragraph is a run-on trainwreck. It's fine to put business into dialogue with full stops. Not everything needs to be stitched together with commas. Honest.