Clues are the collectibles in the game. They are hidden in various places in each chapter. The best strategy to collect as many of them as possible (or even all of them) is to always go in the other direction than the one pointed by the camera - the camera always guides you to the next objective, and the road back, to the left, or to the right will let you enter a side room, hiding important collectibles. Collecting the clues (except one, mentioned in the walkthrough - chapter 10) is not mandatory and doesn't impact the game ending. However, the clues aren't completely separated from the story - when you find something interesting as one character, you can then see as that characters tells what he or she found to someone else during some cutscenes. Some hidden objects can be collected by more than one character. However, since often some rooms are only available for a short while, it is wise to look around thoroughly each time. Totems can be also considered as classic collectibles. However, since they are quite specific, you can learn about them in the next chapter called Totems.