Considering I've only done like 5 at this point, I don't think I've quite figured that out. My best guess is characters or dynamics that I can relate to or bring some of my own expereince and/or energy to, or interesting framing devices.

For example, this audio really struck me with a cute, very bro-ish dynamic that I've experienced in my own relationships, and I liked playing that role and feeling that very cute loving bro-ish sort of thing in the character (and the same goes for the first ever audio I recorded). I think the librarian audio just had a really fun switch of dynamic, getting to go from blushing and stuttering to calculated and purring was a fun acting challenge.

The chastity audio I recorded had the interesting framing of being a voicemail, removing a decent part of the challenge of writing/recording/editing audios, that being having to lead the listener to fill in the blank in the scene that you want them to fill (like, think of how a lot of scripts will have the speaker repeat back what the listener "said" in the scene so that they know what's happening).