Agency is an interesting term which is cropping up more and more in two arenas, employee management and storytelling. In these contexts, it means 'possessing the means of acting,' or more losely, power. Power, of course, is a loaded term, but think of the simple physical definition of 'being able to perform.' Agency is an important concept in storytelling because it's a required of both heroes and villains. Few people can root for a protagonist who is powerless or feel threatened by an impotent villain.
Accountability is meaningless without agency, but it's extracted from people all the time. And that's ineffective management: it happens but it doesn't lead to good outcomes.
LinkedIn: What do you mean by accountability?, 2014-Aug-1 by William Falquero
If you want someone to do something, give them the ability to act. Whether it is budget, tools, time or insights, you have to clearly define the call to action and provide the latitude to do. All too many great performers have been asked to complete stretch assignments without ever having a fraction of the resources to try to get the job done. Agency comes first.