“Unless you are following the dialogue with an action and not a dialogue tag.” He took a deep breath and sat back down after making the clarifying statement. 

“However,” she added, shifting in her seat, “it’s appropriate to use a comma if there’s action in the middle of a sentence.”

“True.” She glanced at the others. “You can also end with a period if you include an action between two separate statements.”

Things I didn’t know

“And–” she waved a pen as though to underline her statement–“if you’re interrupting a sentence with an action, you need to type two hyphens to make an en-dash.”

You guys have no idea how many students in my advanced fiction workshop didn’t know any of this when writing their stories.