So if I'm correct, you start a new paragraph in dialogue every time a new character is the one focused on.
So if character A says something and then does an action that's all one paragraph, but if character A says something and then character B does an action you'd need to separate those things with a paragraph break. But what if character A does something and then both characters are involved in the non-dialogue text?
This is the sample at hand in case anyone's confused: "Time sure flies when you're having fun…" David frowned and for a moment Morgan detected something strange in his expression. "You should be nicer when you talk about your father, he's a good man: he looks after you and it's pretty clear he loves you very much." (both pieces of dialogue are being said by the character David)