So I wondered if anyone else has ever done this? Or, while reading, you don't so much wonder as that character just goes into the book and you KNOW what will happen.
Here is an opportunity to let your imagination run free. What book are you currently reading, or have read recently, and think of someone from another book somehow, magically, being written into your current book. This could be anything from a character from one dystopia book to another, or cross genres. It could also be someone who is 'real' (for example, maybe you are reading a biography) and suddenly Cary Grant is in The Hidden Staircase. What would happen?
Another example -- what would Elizabeth Bennett (of Pride and Prejudice) do if she finds herself in Matilda or House of Furies or Ella Minnow Pea?
What would Katniss Everdeen do if she found herself written into Harry Potter and the Goblet of Fire?
The possibilities abound!

