Action --> Reaction --> Thoughts / Feelings --> Action
Dialog is action, and good dialog deepens tension between characters. To intensify that tension, add reactions and thoughts / feelings. It makes the story more realistic because when we talk to people we react, we have feelings, we think. Many times, what goes on inside is nothing like what's on the outside.
Consider this excerpt from my book Infiltrate:
"Where's Lelea?" Jaak demanded, looking around. "She's the one that scares me. She's the one that'll get us killed."
Jadon wadded her hands into fists and leaned across the table. Jaak turned white. "Lelea knows what to do," Jadon growled.
She wasn't going to admit that she had her doubts too. Sure, Lelea could do the mission in her sleep, but that didn't change her sister's strange behavior. And it wasn't the chip in her brain that made her so unstable. Jadon had run diagnostics on it many times over the last year. No, there was nothing wrong with it...
She sneered at the scub cowering in front of her. "She's been training for missions since she was two, but you haven't even made it through Lunk. Remember, scub?"
Action: Jaak accuses Lelea of likely getting them killed.
Reaction: Jadon wads her hands and leans forward.
2nd Reaction: Jaak turns white.
3rd Reaction: Jadon growls and defends her sister. Arguably, this could be an action, but in this case, I used dialog as part of her reactions to Jaak.
Thoughts / Feelings: Jadon considers whether Lelea really is up to the task.
Action: Jadon throws Jaak's inadequacies back in his face.
All rules are made to be broken. There are times when dialog should be back to back with nothing in between. When do you break this rule? The answer is coming tomorrow.