"But if you try to drive across the creek bridge, the flood waters come across it very fast," I pointed to the downstream side, "and the water can push your car off."
Okay, most readers would be looking for a dialogue tag where the narrative is, but the writer never intended that. The writer has instead placed some amplifying narrative inside the dialogue to relate some action during the the speech. Now, inserting some straight narrative without a tag into dialogue is a good way to indicate action that occurs during the speech, but you can't do it with commas. When you do, you end up with something like a comma splice between dialogue and narrative and another one from narrative back to dialogue. I guess. Well, anyway.
You imbed action narrative into dialogue with a pair of em dashes. The information below is covered in more detail in my little eBook, but a continuing curiosity among writers is that many of them will pay an uncredentialed "book doctor" a thousand dollars to tell them they didn't develop the transvestite's character enough, but will flinch at the thought of paying much less to have their grammar and usage—the things that cause manuscripts to be rejected more often than for any other reason—checked and fixed. Those writers would probably also balk at shelling out ten bucks for Writers' Devils to learn things they would say shouldn't matter to true literary artists anyway, so here might be the only place those folks will ever get this information, where they can have it free and benefit from it with the anonymity that doesn't require them to openly attach any importance to it.
Let's say you want Tom to say something to Dick, and you want him to begin to get up and walk toward Dick at some specific moment as he says it.
Not before he says it:
Tom got up and started walking toward Dick. "If you don't kill this story, you're going to find that employment of any kind is just a little out of your reach."
Not after he says it:
"If you don't kill this story, you're going to find that employment of any kind is just a little out of your reach." He got up and started walking toward Dick.
Not at some indeterminate point during the speech:
"If you don't kill this story, you're going to find that employment of any kind is just a little out of your reach," Tom said, getting up and walking toward Dick.
If you want to pick a particular moment when the simultaneous action begins, action that doesn't interrupt or cause a pause in the speech but simply begins at some specific point during the speech, use a pair of hyphens to imbed the descriptive narrative:
"If you don't kill this story, you're going to find"—he got up and started walking toward Dick—"that employment of any kind is just a little out of your reach."
Here's another one:
Preacher shook his head and said, "You can't settle anything with a gun."
"Oh, you'd be surprised"—Yulin lowered his right hand onto the handle of his holstered Colt—"just how much you can settle with a gun."