What gave Harry the idea of writing in Tom Riddle's diary in Harry Potter and the Chamber of Secrets?
It is clear that if Harry hadn't seen that story from the past through Tom Riddle's diary, he wouldn't have moved ahead in cracking the mystery of the Chamber of Secrets. And he could get all that info only by interacting with Tom Riddle through a written conversation via that diary. But what gave him the clue to write something in a blank diary?
I mean, in the movie, it's not even some accident by which he figured out that writing in it would help. He literally took an ink pen and first dropped some ink and when it disappeared he started writing in it.