Oh so thats what that is!!
So I did this all the time!!! when I was a kid! not as much anymore but when I was a kid I would just have stories playing in my head like so vivid I could see characters and they would have names sometimes I'd even know how many chapters there was and when the story would end. I haven't finished a story since I was like early 20's but when I was a teen I would go hours of watching these stories play out! Some of them would be so good and by the way never "actual books" just what my brain would come up with!! I wrote some of the more memorable ones down but sad thing was once the "story" was finished I never got to see "read" it again. One time as like a test of or to freak a friend out I told her verbatim everything that was happening in a story like word by word. I recall just sitting with her and it was a four hour event. Several times she would stop me and be like you should be writing this down! Now if I can't sleep i'll try to start a story but I only get like 4 chapters in before it goes too fast and I know how it ends or I fall asleep. But as a kid I would have to go through the entire story in my head to even knew how it was going to end. Now I know the name of this!! Lol I just figured my imagination was a little wild.