Part of my role involves PA'ing, I know EA is a bit more in depth but probably similar is:
*Ensuring their wellbeing, e.g. factoring in travel times, you don't want them having meetings they don't have time to get to, or aren't logical e.g. if they're due to meet with 2 different people who are a way away, can it be the same day/consecutive days with a hotel stay - rather than have one, come all the way back for a day then come all the way back
*Things frequently come up that trump an existing meeting that has to be rescheduled due to the urgency of the emerging priority - so being able to send apologies on their behalf and looking for alternatives at the same time
*Helping them prepare for meetings - have they got all the documents they need in the format they need? (E.g. our CEO prefers lengthy documents printed so he can properly read them). Do they need anything booked in e.g. travel or a hotel. Has all the paperwork that was needed for the meeting been received.
*You may be asked to prepare documents for them/proof reading/pulling stats for a meeting they have coming up.
*You may well have to send emails on their behalf so need to ensure they reflect their professional standards - almost always they will be formal, you won't want to be sending casual emails!
*You want to explain how you can make their working day as easy as possible, so they can focus on their role - so doing things like making sure their diary is in order, no clashes etc