Your scene partner says, "Look at those flying dogs."
You answer, "Yes, and they look a lot like pterodactyls."
As opposed to saying, "Those aren't flying dogs. They're pterodactyls."
The rule applies to the freelance writing life, too. No matter what comes out, stay with the flow and continue the writing. Yes and ..., yes and ..., yes and ...! There's plenty of time for no's during the editing process.
Actually, the rule applies to many aspects of life. My life, anyway.
Say yes. Be open. See where the yes takes you.