it's in the note of manual "This is a collaborative experience. Please be kind to others while writing your part." there are no a single rule that force the fiction to a collaborative experience, every player play on his own...
There is nothing that forces anything in the game, including the prompts. So again, you can play it as collaborative as you want. For example, you can get together with friends, brainstorm on ideas and then carry them to your journal.
