Hi, Vieome! Here is a partial, personal answer to your question.

For me, depression is most obviously a disease of the will. It means being mysteriously unable to mobilize myself to do what I want when I want, even things that should be enjoyable. I can imagine doing or participating in something, but I find my emotions and body won’t work with my imagination to act. With that, I find I don’t or can’t care I’m “unable” to act.

That’s the obvious side of my depression. Beneath the surface I’m vaguely aware of a more fearful aspect of the illness, but it’s a master at hiding from me.