First of all, this game has some bugs. Save in multiple slots to prevent bugging the game and not being able to go back to a safe state.
Once you first encounter Miss Sutton, she asks you to open a safe in the library. There are two passages further from where she's been hiding. One is descending, it leads to to the medical examination room. If you go there, there will be a zombie, and in the end of the room, near four mummies (or whatever these are) there will be a bucket of water. Don't take it.
If you do, you will bug the game. After Darien becomes possessed, you play as Sutton, and you have to search for the cure. You cannot use it on Darien if it was him who picked up the bucket, not Sutton. So, make sure she's picking all of the components herself, and then you will get everything right and the cutscene of her coming to Darien with a cure will play as it should (but doesn't if he was the one picking the bucket).