The world in this story mimics the real one. The main character will start as a teen. The dates you see at the beginning of every chapter will represent the time elapsed. The story is basically this, MC finds a diary, and writes in it. The next day, he sees words he hasn't written (your comment), show up in the diary, as if it has responded. Your comments will decide the MC's actions. I'll let a chapter accumulate comments for a while, before choosing a comment on random. You could gain the MC's trust and then have him commit atrocities. You could actually advise him well. You can do whatever you want. The chapters will be in the form of a diary entry.