This quote is a playful way of warning people to be cautious about their actions and words around the person who said it. It means that if you're not careful, the things you do or say might become part of a story or book that this person is writing. In simpler terms, it's like saying, "Watch out, or you might become a character in my story."
Imagine if you were hanging out with a friend who's a writer, and they said this to you. It's like a friendly reminder that your interactions could inspire something in their creative work. So, if you do something funny or interesting, it might end up in their book, perhaps as a character or a memorable scene.
Overall, this quote is a lighthearted way of saying that the people around the writer are a source of inspiration, and their actions and words could find their way into the writer's storytelling. It's a fun way of highlighting how writers often draw from their real-life experiences to create captivating stories.