Who is writing this story?

Where does the story come from ?
The other night I was standing in the shower and I could clearly hear the dialogue in my head. A character said something and another replied with the perfect come back.  I giggled and thought to myself “that’s a good one” .
They were fabulous, I just stood back and listened to them engaging with each other.  Entertained and captivated,  It suddenly occurred me … I should write all this down !