Your job is to reduce the number of distractions or minor tasks that your manager has to handle. You should not expect every need to be clearly outlined for you.
For example, your employer should not have to explain that they should not be interrupted during a meeting with the CEO. You should anticipate this need.
Anticipating managerial needs is an essential skill that every assistant should learn. Anticipating needs requires using common sense with each task. For example, if you schedule a flight and book a hotel, the need for printed boarding passes and a map with directions should be anticipated. By paying attention to the demands placed on your manager and anticipating needs before they arise, you will make yourself an invaluable employee and respected assistant.