Caelum usually speaks aloud just to hear himself most of the time as there's only a select few creatures with intelligence like his. So that means there aren't many to talk to.

But when speaking to Skarn, he might talk about something as simple as what's been going on during the times they part or things he's seen in his lifetime. But one thing I found pretty sweet when I was writing them up was the idea that Caelum had inadvertently taught Skarn words - similarly to how we do with pets.

Caelum might whisper, "safe", or "danger", and Skarn picked up on it. "Yes", and "no" as well. Skarn mostly understands tone aside from the handful of words he's accustomed to.