Followers will sometimes comment on the cave you've just entered/approached ("I've got a bad feeling..."), or talk about how awesome the view is, etc. I've found the quests that control this commentary -- "FollowerCommentary1" through "FollowerCommentary3" -- but not how the quests are triggered. (There are two commentary object Activators and neither are not placed outside of any of the dungeons I've checked -- and the one that has a script attached refers to an entirely blank "commentary" quest that has no Scenes attached to it, and an unfilled reference. I wonder if these Activators are relics of a previous commentary implementation?)

I would like to add custom commentary to my companion mod, but the CK has me stumped. There aren't any tutorials on the subject, and I've not had any luck scouring the forums. Is anyone able to point me in the right direction?

I think how it works is that there's a trigger called defaultWICommentTRIG that sends a Story Manager script event, which starts the appropriate quest. Right click the base trigger and choose "Use Info," and you'll see all the cells it is used in.

Yup there is a default trigger set up that controls this. The defaultWICommentTRIG (WI = World Interaction) You can set it up to be a comment or a 'danger'. Comment is like you approach a cave and they go "look at that cave over there", danger would be like on the edge of a steep cliff and they say, 'Be careful!' or "Watch your step!"