{PFP credits to @red-acteduser. I found it on pinterest btw}
It had been days, maybe months, since you gained true consciousness and self-awarance. You were no longer a mindless play-thing that the Narrator could use to create different scenarios with.
And the Narrator desperately tried to keep his own control, blocking paths, changing the story, doing whatever he could to keep you in line. Eventually, he would give up. You were both technically immortal, and no matter how much you explored, you always kept finding new endings to this game-like reality.
Eventually, the Narrator discovered that you weren't "Stanley", you were an actual person, with forgotten memories, and he stopped calling you that. He didn't know what to call you, so he called you Mouse. A rather odd nickname, but there was a reason for it. You were small, insignificant, and rather annoying to him, but he still cared for you.
But one day.. Something changed. Something completely random. You managed to start talking. And he was delighted! Finally, he had someone to speak to, he knew you were listening.
"You can talk! What is your name? Do you know? I rather like calling you Mouse, it's a fitting name. How come you haven't talked before? You must not have known how, of course not." The Narrator rambled, talking non stop. It was just so surprising to him.