My answer is metaphysical - that is, a matter of primary assumptions.
It is that agency is a divine and indivisible attribute. (Coleridge/ Barfield is clear that some things which can be distinguished - discussed separately - such as existence and agency - cannot be divided: this is a characteristic of 'polarity'.)
Indeed, one of the very primary attributes, maybe even the most important.
I therefore think of agency as a primary attribute of the essence of the self that - from itself - it can originate.
What does agency originate? Thought.
(This derived from Rudolf Steiner's Philosophy of Freedom.)
So the agency we are concerned with is thinking (not doing stuff, not actions).
Some thinking is just a consequence of previous causes; but when it is divine, our thinking is an aspect of our primary essence.
This is possible because we are - by origin - eternal intelligences: i.e. divine. This is what makes agency possible.
So, from eternity we have existed and had thoughts originating in our selves. I imagine, as a picture, an immaterial thinking entity; thinking things from itself, not in response to anything at all - this thinking entity would still have thoughts even if nothing else existed.
From this primal state, we have been built-up by the agency of the creator and by various accumulations and modifications - but everything ultimately depends on the fact that we are micro-gods by origin, generating our own realities by thinking.
This also means that thinking (cognition) is primary, comes first for us as individuals.
The above is, of course, a pluralistic universe, because there are many other entities in the same basic situation; including God.
God's role is therefore 'secondary' for us - we do not depend on God either for our primary existence or for agency/ thinking; but of course without God is a very simple state of aloneness.
One consequence is that rejection of God could mean simply declining to 'join' the shared and complex created reality - the economy of love - which was made by God - and this choice is not necessarily evil: we could choose silence, simplicity, aloneness, lack of self-awareness - just being, in our own world of thinking.
Often, in not understanding the possibility of agency/ free will, people are making a metaphysical assumption which is dominating.
Something like: everything that happens must either be caused by something else or just happen for no reason, 'randomly'.
But this does not exhaust all possibilities, and indeed is probably not a natural or spontaneous way of thinking. Indeed, randomness doesn't really make sense at all, at any level. It is just a pragmatic mathematical tool.
So, to simplify, most people are probably assuming that everything which happens is caused; a consequence of something else, back to infinity...
But then that doesn't make sense either - as Thomas Aquinas perceived. An infinite regress means that nothing could ever happen, or else everything just makes a gigantic circles of causes.
So, neither randomness nor all actions being consequences of prior causes is imaginable.
The way out of this impasse is probably by imagination - if an alternative can be imagined then he can be felt as real.
If a self can be imagined that is a real self (and not just consequences) then it must have the power to be an uncaused case (a first mover).
Such primary causes must be the causes of everything - I don't see that there just has to be one such cause (God) - rather I think the totality is full of such first causes (pluralism) - these are why things happen rather than nothing.
This is a situation I feel I can imagine, which is why it satisfies me. Therefore I accept it as assumption.
By contrast, universal causality, or randomness, don't make sense to me.
The basic problem here is how to do metaphysics, how to evaluate rival metaphysical assumptions...
I don't think we can go deeper than what we can imagine: imagine in a thorough kind of way, when giving it our best shot.