you can skip the dialogue at the start of the game by holding down the escape key and you can speed through all the other dialogue just by holding space or enter.
the shadow appears in three set rooms throughout the game that are a bit scattered throughout the house. it will always stop chasing you if you reach the room the mirror in it.
the house becomes very dark after a certain point in the story and then they return to normal after another point, but other than that they don't change.