Hot damn, that's why! I've been messing around with follower gear and inventories lately. One time after restoring gear to my actor he suddenly drew a bow when he went into combat stance. I was like... where the hell did that come from?!
Probably going to have to rewrite all of the vanilla quests to get some of the quality and interaction I want. *sigh*
The sad truth is, it's not only a complete new follower quest and dialogue you'll have to set up. If you're planing on voice acting, there's more work going to be put in it.
I think it's easier to just copy the DialogueFollower and DefaultFavorFollower(or how was it named?) Quests and delete anything you don't need and modifie anything you will need. Make sure to redo every script inside thoose quests and you're good to strat from there. Doing everything from the scratch will cause hours of headache.