When the hard part of what you are trying to do is in the many small details, you can say "the devil is in the details." Example: "I thought I would be able to write that article in two hours, but it ended up taking me five. The devil was in the details." The devil is known for always make life difficult for man in many small ways. Example: "I can sketch a basic outline of the plan for you and it may look very simple, but the devil is in the details."