One of my books features an immortal protagonist and I’ve as such thought about this quite a bit. More than the answers already provided here, what I found interesting as a writer was the balance I needed to find between making an immortal detached from mortal values while still being engaging to mortal readers.

Said as a pithy question, if you can outlive everyone’s decisions and mistakes, what would it take to make you do anything at all?

I wrote a story that features such an entity and what was interesting about it to me was how even the slightest glimmer of life beyond their void would lead to an all-consuming desire to experience “living” again.